Allen Iverson's got a huge game coming up ... and here we are talkin' 'bout practice!!!

'The Answer' hit the courts at Chelsea Piers in NYC Friday to shoot around with other former NBA stars -- like Kenyon Martin -- and get ready for the tip-off of Ice Cube's BIG3 league Sunday at the Barclays Center in Brooklyn.

He even took some time to cross over actor Michael Rapaport just for fun ... like he once did to MJ.

Iverson's 3-on-3 team -- 3's Company -- will take on the Ball Hogs in their first game ... who are captained by Brian Scalabrine and Josh Childress and coached by Rick Barry.

We also asked Iverson if he has any words of wisdom for top pick in the NBA Draft, Markelle Fultz ... his advice was short but sweet.

ALWAYS BALL HARD
TMZSports.com