What is it that you want?

A viable tank that can also heal? That'd be a pally (or a druid....).

A fun third character that can act just like a DPS caster and also heal? That'd be a shammy, priest or druid.

With a team of three, I expect you'll be questing most of your time (rather than instancing) so if I were in your shoes I'd probably add a shammy. That would synergise pretty well with the mages but offer you the flexibility to heal and even take a few hits (I hesitate to use the word tank) when needed.

You'll also get the shammy pets at higher level, and it sounds to me like you don't actually need a full-time tank or a dedicated healer.

My second choice would be a druid.