I pretty sure the answer from a lot of people is going to be "A third shaman" which is a very good choice. But really it depends on what you want to do with them long term. You could easily team them up with a DK or some other tanking class to make taking on PvE content that little bit easier, which would then also make getting heroic groups with your 3 team easier as well as tanks are always in short supply.

So I guess my answer is either a tank (I said DK as they are 58 oce out of starting area so easy to get up to that level) or another shaman. Im presuming your elemental at this point so Boomkin or Warlock would give you nice spell casting buffs as well, with the boomkin being another healer/resser like your shamans.