A lock, mage and priest would be pretty hard for a first timer.

3 of the same class is generally the easiest way to start. You're kind of gimped with only 3 because you aren't going to be able to solo dungeons 'easily'.

Depends what your goals are I guess and totally understand about being short on $$$ believe you me!
You can still have a lot of fun with 3 but let me tell you having 5 is awesome and well worth it.

If I was in your situation I'd go with a healer-tank-dps. Just having a tank+healer will take you far. Maybe try a Paladin+2 druids. Spec the druids both for DPS since they'll have PLENTY of healing power while leveling up. When you get to higher levels you can change one druid to a healer and leave the other as your DPS.

But you might think of a long term goal like I did. Started with 2 accounts and already had an 80 paladin. Dual boxed 2 sets of druids and by then had saved enough to buy 3 more WOWs+transfer fees. Something to think about =)