NFL superstar Frank Clark is now facing up to 3 YEARS behind bars ... after prosecutors hit him with a felony charge of possession of an assault weapon Friday, TMZ Sports has learned.

We broke the story ... Clark was arrested on June 20 -- after cops say they saw an uzi in his Lambo SUV during a routine traffic stop.

Officers say the firearm was sitting inside of an open duffle bag in the ride ... so they arrested the 28-year-old Chiefs pass rusher.