Jermaine O'Neal says Ice Cube's BIG3 league is NO JOKE -- telling TMZ Sports, "We ain't signing up for it to be a clown show."
O'Neal -- along with a slew of ex-NBA stars -- have signed up for the 3-on-3 league ... and O'Neal says he's dead serious about competing at a world-class level.
You can see in the video, the 6-time NBA All-Star looks to be in pretty good shape ... and says he and his old running buddy Bonzi Wells are in it to win the whole damn thing.