There are several type of Class 3 items that are restricted by the National Firearms Act.
Each state can impose additional restrictions on the sale, purchase, and transfer of class 3 firearms in addition to the compliance that is required with the national Firearms Act.
In Washington you can own the following items that are regulated the the National Firearms Act
Silencers Any Other Weapon (AOW)
Destructive Devices (DD)
In Washington you cannot own the following NFA restricted items.
Short Barreled Shotguns (SBS)*
Short Barreled Rifles (SBR)*
NOTE:* Not legal to own or possess parts that can make these firearms unless these items were legally acquired prior to July 1, 1994 and be in compliance with federal law or machine Gun, Short Barreled Shotgun, or Short Barreled Rifle but be possessed by by peace officer for official duty, armed forces, or person in compliance with NFA who has undergone Fingerprint and background check who in engaged in the production , manufacture, repair, or testing of Machine guns, SBR, or SBS
Follow this link to find out more about Washington and NFA restrictions on Class 3 Firearms
Note: Although you can own a silencer, it may not be used on a gun. Also for other class 3 items a state license is required in addition to the requirements under the NFA