Quick reads... ridiculously loose table with about 6 people seeing every flop and almost no PFRs (< 3%). That's all I knew so far.

Party Poker 0.50/1 Hold'em (9 handed)

Preflop: Hero is UTG with , . MP3 posts a blind of $0.50.
Hero calls, UTG+1 calls, MP1 calls, 1 fold, MP3 (poster) checks, CO calls, Button calls, 1 fold, BB checks.

Flop: (7.50 SB) , , (7 players)
BB checks, Hero bets, UTG+1 calls, MP1 calls, MP3 raises, CO calls, Button calls, BB calls, Hero 3-bets, UTG+1 folds, MP1 calls, MP3 caps, CO calls, Button folds, BB folds, Hero calls, MP1 calls.

Turn: (14.25 BB) (4 players)
Hero bets, MP1 calls, MP3 calls, CO calls.

River: (18.25 BB) (4 players)
Hero bets, MP1 calls, MP3 calls, CO calls.

Final Pot: 22.25 BB

I usually fold AXs lower than ATs up front, but here, the table was simply too terrible and I couldn't resist it. It certainly paid off here. My question is how to tackle the turn when my draw hit.

Here's what I was thinking... if I check and it gets checked around, I'd lose out on 3BBs. I'd absolutely hate that result! And since MP3 loves his hand so much, there's a good chance he'd still raise me on the turn, allowing me to throw in a 3rd bet. Obviously this failed since the flush card scared him.

Was this an easy check-raise situation?

meh-- i dont mind your play at all. The way i look at it, a CR might have scared off someone who wouldnt be willing to call 2 bets cold, so it might not have been worth a WHOLE lot more bets. On the other hand, having it check around on the turn would have been nothing short of DISASTROUS. Also, there is also a chance, with action like that, that someone else was on a flush draw also, which means that you get 3 bets in on the turn at LEAST... so all told, i think you played it fine.

this is fine as MK said. If you check and the CO bets and you check raise, you might force out the other two and only get 2 BB on the turn while announcing that you have the fluhs. You would then lose out on 2 more BB on the river. By betting out, someone might raise or you make it hard for them to get away on the turn or the river.