The Beretta 92FS appears first in "Broken Arrow" in flashbacks in the hands of American soldiers and ARGUS Director Amanda Waller (Cynthia Addai-Robinson). Deadshot uses one during the hostage rescue mission in Kasnia throughout "Suicidal Tendencies" as his sidearm, which makes sense since he used to be with the military. ARGUS agents in China use them near the end of "The Fallen" in a flashback sequence.
Colt M1911A1
ARGUS Agent Maseo Yamashiro (Karl Yune) uses a Colt M1911A1 in Hong Kong as his main sidearm, first seen in "The Calm", when Oliver was held captive in the territory for five years. The pistol is also seen in Lian Yu throughout "The Return". Oliver uses in a flashback sequence in "Broken Arrow" in a continuity error.
Crossman 2240 CO2 Pistol
Roy uses a Crossman 2240 CO2 Pistol to knock out Nyssa with a tranq dart at the start of "The Offer".
Glock 17
The Glock 17 is seen throughout the season in the hands of various characters.
SIG-Sauer P226s are used by both Diggle and Lyla throughout "The Brave and the Bold". Maseo Yamashiro and Oliver briefly use it in Hong Kong in "Nanda Parbat" with the latter using it briefly again at the end of "This is Your Sword". Captain Lance uses it as his main sidearm throughout the season.
SIG-Sauer P226R
SIG-Sauer P226Rs are seen with China White's Triad 49ers in a flashback in "Midnight City". Maseo briefly uses it in Starling City briefly in "The Return".
Taurus PT92AFS
A Taurus PT92AFS with faux pearl grips is seen throughout "Left Behind", used by Danny Brickwell (Vinnie Jones) as his main sidearm in the season. The Taurus in this episode is the same one that is used by Sam (Jared Padalecki) in Supernatural, since Arrow and Supernatural are both supplied by the same armorers (this gun was also used regularly in Smallville).
Walther P99
What appears to be an unidentified Walther P99 variant is used by an arms dealer tries to shoot Oliver in "The Calm" (S03E01). From the shape of the trigger guard, it appears that this is a 2nd generation model such as the AS or QA, however it is never seen clearly enough for a positive ID.
Revolvers
Smith & Wesson Model 36
A Smith & Wesson Model 36 is used by Malcolm Merlyn prior to his League of Assassins training in "The Uprising".
Submachine Guns
Cobray M11/9
Rogue ARGUS agent Mark Shaw (David Cubitt) uses a Cobray M11/9 in "Cordo Maltese". M11/9s were seen with Danny Brickwell's men, first seen at the start of "Left Behind". An ARGUS plainclothes agent in China fires it at the Yamashiros and Oliver near the end of "Nanda Parbat". A few are seen with gangsters without shoulder stocks at the start of "This is Your Sword".
Heckler & Koch MP5K
Heckler & Koch MP5Ks are seen with some of Danny Brickwell's men, first seen in "Midnight City". A Triad 49er under Chien Na Wen uses one in "The Return". An ARGUS plainclothes agent in China fires the MP5K at the Yamashiros near the end of "Nanda Parbat". One is used by an ARGUS commandi in a flashback in "Public Enemies". Lyla and Diggle use them as their main weapons after being deployed in Kasnia throughout "Suicidal Tendencies".
Heckler & Koch MP5A3
The Heckler & Koch MP5A3 is used by the SCPD's Emergency Services Unit as their main submachine gun, first seen near the end of "Public Enemies".
Heckler & Koch MP7A1
Some of the mercenaries who met with Mark Shaw in Cordo Maltese use Heckler & Koch MP7A1s in "Cordo Maltese". Armed men working for Cooper Seldon use them near the end of "The Secret Origin of Felicity Smoak". An ARGUS commando hunting for Oliver and Akio was seen using one in a flashback in "Public Enemies".
Kimel AP-9
Kimel AP-9s are seen in the hands of Brickwell's men near the end of "Left Behind".
Steyr TMP
A Steyr TMP is seen used by a gangster at the start of "Take is Your Sword".
Assault Rifles
AK-47
AK-47s are seen with some Kasnian mercenaries throughout "Suicidal Tendencies".
AKS-47
AKS-47s are seen with some Kasnian mercenaries throughout "Suicidal Tendencies".
AKS-74U
An AKS-74U is used by a Triad 49 during an ARGUS raid in "Left Behind".
Colt M4A1
The Colt M4A1 is first seen at the start of "The Brave and the Bold" at the hands of ARGUS tac team assaulters and other agents, being their main primary weapon. Some of Danny Brickwell's men use them in "Uprising". Most of Werner Zytle/Count Vertigo II's men are armed with it throughout "Canaries". Diggle uses this during a trip to Nanda Parbat in "Nanda Parbat" and in "This is Your Sword". US Army soldiers use them throughout the season.
IWI MTAR-21
One of the armed men working under Cooper is armed with an IWI MTAR-21 carbine in "The Secret of Felicity Smoak".
Norinco QBZ-97
Norinco QBZ-97s are seen in the hands of Michael Amar's (aka Murmur) men throughout "The Offer".
Norinco Type 56
Majority of the paramilitary mercenaries Oliver, Roy and Diggle confront in "Costo Maltese" are armed with Norinco Type 56 assault rifles.
A Zastava M70AB2 is seen in a debriefing photo at ARGUS HQ at the start of "Suicidal Tendencies."
Shotguns
Daewoo USAS-12
One of Cooper's men uses a Daewoo USAS-12 near the end of "The Secret of Felicity Smoak".
Franchi SPAS-12
One of Cooper's men uses a stockless Franchi SPAS-12 near the end of "The Secret of Felicity Smoak".
Franchi SPAS-15
A Franchi SPAS-15 is used by a SCPD ESU officer near the end of "Public Enemies".
Remington 870
Remington 870s are seen throughout "Uprising" with Danny Brickwell's men.
Sniper Rifles
IMI Galil "Galatz"
An IMI Galil "Galatz" is used by Diggle as a sniper rifle at the start of "Public Enemies".
Nemesis Arms Vanquish
The Nemesis Arms Vanquish is seen handled by Maseo and Oliver during the latter's time in Hong Kong in "Sara". The former later uses it again in "Left Behind".
Remington 700 with Choate Super Sniper Stock
A Remington 700 outfitted with a Choate Super Sniper Stock is used by Deadshot when he covers for the squad and the hostages near the end of "Suicidal Tendencies".
Machine Guns
Daewoo K3 Paratrooper
A Daewoo K3 light machine gun modified to resemble a M249 was used by a mercenary in Costo Maltese in "Costo Maltese".
GD GAU-22/A
A GD GAU-22/A machine gun is seen with a Lockheed Martin F-35 Lightning II used by the League of Assassins near the end of "This is Your Sword".
GE M134 Minigun
What appears to be a GE M134 Minigun is seen near the end of "Corto Maltese" under top-secret Queen Consolidated Advanced Weapons blueprints.
M60C GPMG
Four M60C GPMGs with EOTech sights are rigged to fire via remote control in "The Secret Origin of Felicity Smoak".
Rocket Launchers
RPG-7
The RPG-7 is seen twice in "The Calm", from an arms shipment seized by Diggle and from one used by Werner Zytle/Count Vertigo II (Peter Stormare). A RPG-7 equipped with iron sights is used by one of Cooper's men near the end of "The Secret Origin of Felicity Smoak".
Grenades
Smoke Grenade
A smoke grenade used when Danny and his men invade the mayor's office at the start of "Midnight City".
Others
Oneida Eagle Kestrel
The Oneida Eagle Kestrel returns as the main bow used by Oliver in the season.
Recurve Bow
Roy uses a recurve bow as his personal bow in this season when he's in his Red Arrow costume. Lacroix also uses a recurve bow as his main weapon throughout "Sara" (S03E02). Thea uses Roy's bow and gear near the end of "My Name is Oliver Queen".
Reflex Bow
Carrie Cutter aka Cupid (Amy Gumenick) uses a Scythian style reflex bow as her main weapon in the season.
PSE Chaos AD
Customised Precision Shooting Equipment Chaos AD compound bows with a black finish are the standard weapons used by members of The League of Assassins. Nyssa al Ghul (Katrin Law) uses one again in the season, first seen near the end of "Corto Maltese" (S03E03). Thea uses one after being trained by Malcolm, first revealed in "The Climb" (S3E09). Malcolm uses it again in the season.
Bow
Oliver uses the bow he used back at Lian Yu, first seen in "The Brave and the Bold".
Martin Saber Take-Down
Oliver uses a Martin Saber Take-Down recurve bow with a full black finish given to him by the League of Assassins after he grudgingly joins them at the end of "The Fallen".
Shotgun Armory
A SCPD armory in the Glades has a variety of shotguns in "Uprising".