It really depends on the kind of army you want, with 3 vampires you'll pretty much be stuck relying upon them to with you the game by eating through your enemy's units with them at the head of a horde of cheap ghouls or the like. 1 Vampire allows you to take more of a balance army list rather than relying upon your heroes, but you have to play smart with undead to win (Alot of times I'll place 2-3 blocks side by side and march them up the field with a forest or ruin beside them when they reach the enemy and my blood knights with vampire equipped with blood drinker and always strike charging anything stupid enough to try to flank on that side, or flank charging down his entire army which is hung up on my line of ghouls). Although some people I've noticed enjoy many small units of undead to maximize the usefulness of their heroes, make sure blood knights, etc are deployed last, and to screw with where the enemy overruns to.
All in all I'd say try taking 2 and learn how your army maneuvers, etc. If you decide you prefer to win through characters you can always add another, but you'll probably get the most experience and tactics with 1-2 which will help you greatly playing/against any army.