First, keep your vamps off the line of scrimmage at all times. If you lose vampires, you're going to be in trouble.
Second, don't plan to use too many at the same time; four on the pitch is generally enough, at least at the lower TV levels. Your plan to use three is good. Equally, always ensure that you have at least one re-roll per vampire at the start of the game.
On ball retrieval, I have happily used one vamp and one thrall - you really want the vampire to have the ball. The couple of times I've had a thrall as my ball carrier, it's been a panicked drive, really uncomfortable.
I play really, really aggressive on defence. 3 thralls in the middle, then four more, one in each wide zone, only one square back. behind each of the 'backfield thralls', place one vampire. That means that wherever the ball is (so long as it's near the halfway line), I can blitz the ball carrier with some good gazing.
Oh, and my experience isn't exactly phenomenal, so maybe take that with a pinch of salt.
