One of the Soviet security guards appears to use a Makarov PM when cornering the kids in the Starcourt food court. Possible Makarovs are also seen when Robin and Steve are captured in the previous episode
Tokarev TT-33
Tokarev TT-33s are used by Soviet security forces and soldiers, often suppressed. Chief Hopper (David Harbour) and Nancy Wheeler (Natalia Dyer) both make use of TT-33s taken from incapacitated Soviets.
Smith & Wesson Model 66
Hawkins Police Chief Jim Hopper (David Harbour) once again returns to using the Smith & Wesson Model 66 revolver as his duty weapon in Season 3. Hopper loses the Model 66 in "Chapter 5: The Flayed" where Grigori acquires the revolver.
Shotguns
Remington 870
Nancy Wheeler (Natalia Dyer) finds a Remington 870 in Chief Hopper's shed and uses it against the Mind Flayer during the assault on Hopper's cabin in "Chapter Seven: The Bite".
Soviet security forces often carry the anachronistic Zastava M92 carbine while guarding shipments at Starcourt Mall, presumably as a stand-in for the contemporary AKS-74U.
"AKMSU"
Curiously enough, a custom AKMSU appears as Chief Hopper (David Harbour) appropriates one and uses it in the season finale "Chapter Eight: The Battle of Starcourt" (S3E08).
AK-47
Soviet soldiers are seen armed with Type III AK-47 rifles. This is anachronistic for Russian soldiers to carry an outdated version of the AK rifle in 1985 as the AK-74 was adopted as the standard rifle.
M16A2
M16A2 rifles are used by U.S. soldiers in "Chapter Eight: The Battle of Starcourt" (S3E08) as they storm the Soviet base. The rifles have large "Maglite"-style flashlights taped to the handguards. While technically not anachronistic for the year in which Season 3 is set (1985), the M16A2's use by U.S. soldiers is unlikely; in 1985, only the U.S. Marine Corps had adopted the M16A2. The Army did not adopt the M16A2 until 1986, and it did not begin reaching any Army units until 1988.
Machine Guns
M60D
UH-1H Hueys are seen with M60D Machine Guns on the M23 mount as they arrive at Starcourt Mall in "Chapter Eight The Battle of Starcourt"
General Dynamics M197 Vulcan
What is probably meant to be a M197 Vulcan cannon is seen mounted on a AH-1 Cobra as reinforcements arrive at the end of "Chapter Eight: The Battle of Starcourt".