I havent tried this setup myself but if you ask me you will be better off having all 3 as rets whilst leveling. At least with dual specs coming in next patch though u can have them specced like u say on the second spec then switch to that if you want to attempt an instance or grab 2 extra dps and go to an instance. I can say though that 3 healers is good fun in AV, people have a really tough time touching you