Stephon Marbury 's better off in the BIG3 ... but is in such crazy-good shape, he could hoop in the NBA too -- so says Al Harrington, one of Starbury's BFFs.
Al knows what he's talking about ... dude's a 16-year NBA vet and reigning BIG3 champ -- and, most importantly, chats with Marbury all the time.
Harrington was literally seconds away from hitting up Stephon when TMZ Sports found him in L.A. -- and asked if he seriously thought the 40-year-old could still ball with the best.