Tom Brady, Rob Gronkowski ... AND Julian Edelman in Tampa?!?!

Gronk says it's all possible after Edelman's retirement from the Patriots ... explaining he believes there's a "69% chance" Julian's a Buccaneer in 2021!

The future Hall of Fame tight end made the revelation at LAX on Monday night -- just hours after Julian hung up his Patriots spikes -- saying he believes his former New England teammate ain't done with the NFL yet.

"He'll be back," Gronk said of Edelman. "He'll be back. Yeah."