I think three is my tops this xpac, but I have more faith in healers than tanks. I've gone through *five* or maybe even six tanks in a single heroic stonecore.
My paladin friend chew through 4 healers in Stonecore. 1 left, cause he didn't like the gear Other left cause of a wipe. Third one quit as well, cause couldn't handle the healing and the 4th had no problems healing everyone.
Usually I heal instances, so I go through more tanks then healers (4 or 5 in 1 instance max.). The times I tank instances, I've had my gf heal ;-). Can't blame the healers tho, they usually get the blame for DPS standing in the fire and ignoring game mechanisms.
I'm the healer. I don't leave. I don't get kicked. And I don't kick tanks. Ever. Even if he's made of paper. I WILL keep them alive. I'm stubborn like that. Grrrrrr. If you want the job done decently, you've got to do it yourself
On my alt, I've also never seen a healer quit or be kicked, though sometimes I do get a bit nervous at getting very little healing but can do nothing about it as I'm a warlock. Healthstones ftw here. I do try and help wherever I can if I see the tank or the healer struggling. A shadow fury here, a deathcoil-followed-by-a-fear there to get the target out of the AoE and have it rooted in the middle of nowhere helps. Or a banish if there's an elemental. I even bring bandages, just in case. Healers need to be treated well, especially when they're not so experienced yet.
Since I've reactivated my shadow priest recently, I often need to suppress the urge to assist with healing - though I just can't leave any of the nastier debuffs undispelled. Some healers try to frantically heal through stuff instead of just hitting their dispel magic button, lean back and relax.
In the ZG heroic on the PTR it is almost always a given that you will go through 2-3 healers on the first boss.
Too many of them don't understand that the majority of the damage on that fight will be link damage and maybe the channeled spell if no one is interrupting. They instead try to conserve mana through what is the phase where they should be topping people off asap, resulting in everyone dying when they get linked the second time.