Ultimately, play whatever seems to be the most fun for you.

I'd caution that more then one toon who does their damage via melee is a lot harder then having three ranged toons or a melee and two ranged.
This is because, once the toon starts to auto attack in melee range, follow breaks.

Three druids is a good option, because 3x Rejuvenation and 3x Regrowth, eventually 3x Lifebloom too, is a lot of fast to cast heals which provide a good boost.
Down the road, you could spec one of the druids Feral, if you ever needed a tank.

If you plan on doing instances, tanks and healers are the most difficult to acquire for a group, DPS toons are a dime a dozen.
Covering the tank and healer roles ensures a lot easier time in doing instances.
Balance druids and Elemental shamans can heal very effectively, plus as a dps spec they dish out good damage for leveling with.
If you end up doing heroics down the road, you might spec one of them to be a pure healer, but for leveling you'd rather have an effective dps who can respec healing down the road, then a strong healer (say Paladin) who would be a second melee dps while leveling.
The more roles you take, the more flexible the group will be, but the harder it will be to manage the differing abilities effectively.
If you plan on pvping with the team, all of the same class is much easier then any form of mixed classes.
All ranged damage dealers is also much easier then any melee toons in the mix, for the pvp environment.

There are a ton of options to play with.