Given the fact that two players called, I doubt you'll get both to fold by raising this spot. I'd rather just use the awesome odds
you're getting on your money & call.
The trick is, while raising with a flush draw is a semi-bluff, its still a bluff. You still need your opponents to fold to make it profitable. So all your questions can really be answered by that simple fact: When to raise? When you can get them to fold most of their range. How much to raise? A raise that is large enough that it has fold equity, but small enough that doesn't have to work every time.