Ice Cube isn't even a tiny bit upset his BIG3 player/coach Chauncey Billups is leaving the BIG3 for a front office job with the Cleveland Cavs ... telling TMZ Sports the league is SUPPOSED to be a pipeline back to the NBA. 

Cube is in NYC where he's gearing up for opening day -- THIS WEEKEND -- and we asked which BIG3 team is the favorite to win it all. 

That's when Cube brought up Chauncey ... and maybe broke some news?!

"It was the Killer 3's (led by Chauncey and Stephen Jackson) for a minute but since Chauncey took that job, it might change."

It's been reported that Chauncey's in talks with the Cavs -- but nothing has been confirmed publicly ... at least not yet. 

Cube also talked Allen Iverson -- telling TMZ Sports the NBA legend has been working out like crazy and he's "ready to go."

#CantWait