If Lamar Odom wants to join the BIG3 league, Ice Cube says he's got an invitation -- there's just one catch. 

Cube touched down in L.A. -- getting into one of the biggest limos we've ever seen -- when we asked how he felt about Odom expressing interest in his 3-on-3 league. 

"I love Lamar man. I'd love to see him play again."

The issue ... "Dude's just gotta get in shape."

If Lamar can get his fitness together, "We'd love to see him play."

Cube also says he's fired up for the 4-point shooting contest with LaVar Ball at Staples Center on Sunday -- telling TMZ Sports he's 100% down as long as Ball shows up. 

The good news for Cube ... LaVar formally accepted minutes ago -- and says he's looking forward to "tasting that ass" after he beats the rapper. 

Seriously. LaVar actually said that.