I have a light blueish grey pinstripe 3 piece with horn buttons that i wear a lot...i get compliments every time i wear it...never fails. its all about feeling good in it. it could be a normal 2 button navy suit, and if you dont feel comfortable in it, it aint gonna look good.
All my suits since childhood has been 3 piece suits. I didn't wear 2 Piece till I started working. Never felt out of place even when I was in late teens and early 20s. Mostly receives compliments for them.
When is 3 piece appropriate? When awake, I reckon. I've worn one almost everyday at work for the last few years, even when I'm mostly keeping to myself in the office. It goes without saying that the extra layers are appreciated on a chilly day, but I like the fact that you can lose the jacket on a hot day and keep the waistcoat, and still be a bit coordinated and dressed up.
The first suit I bought, in fact, was three piece. A horribly overpriced, dreadfully ill-fitting charcoal stripe Gucci number. At least I took something positive away from the experience! Buying any two-piece suit now would seem like I'm missing out on a vital component.
I wear a three piece anytime the mood calls for it. You may get some stares from people who think that you shouldn't be so "dressed up", but it's all up to you. If you can pull it off, then more power to you.