Ice Cube is one step closer to adding "Agent 0" to his BIG3 league -- with Gilbert Arenas telling TMZ Sports he's seriously considering joining up.

35-year-old Arenas was promoting his Life Water company at 7/11 in L.A. this week when he told us he's been in talks with Cube's 3-on-3 league about signing on for next season. 

In fact, Arenas says he's hoping the two sides can work out a sponsorship deal where his beverage company can be the official drink of the league. 

But even if that doesn't work out, Arenas says he'd still consider lacing up -- most likely because he has friends like Kenyon Martin who are having a blast getting back on the court. 

As for Gilbert, he's been staying in basketball shape -- constantly posting workout videos from the gym ... and it's impressive stuff.