That's the conundrum. As far healing itself goes, our healers are actually pretty balanced. All three of them have a healing kit capable of meeting the healing requirements in the game. The other members of your group don't really care how you do it; be it barriers, direct heals, or HoTs. All three healers can also put out decent amounts of damage, and once again, the party does not care how you do it.
If we want to achieve balance, we have to look at what is actually unbalancing them. The way to achieve balance is that you deliberately have to give each healer a weakness that is compensated for by the strength of another healer, but you also have to do this in a way that they can still function by themselves in solo content, or in group content where they are the only healer. The glaring issue right now is that WHM has no strengths over SCH and AST. Both of them can meet healing requirements, while at the same time bring utility to the group.
Let's look at the utility of AST and SCH: Both of them can increase the raid's speed. Both of them can increase the raid's damage. Both of them can increase the raid's defense.
Can you see where I am getting at here? Two of our healers can do all three of those things, and one of them can't do any of it. One healer should be able to increase raid speed, another should be able to increase raid damage, and the other should be able to increase raid defense. Right there you will have distinctive strengths and weaknesses each healer brings to the table. Since the meta will always favor damage, the latter is the weakest of those three so long as heals keep them up and running, so that particular healer is going to need to have a higher pDPS than the other two.
If the devs keep fiddling around with healing and damage potencies, they will continue to chase their own tail. If they keep giving healers more ways to heal, they will continue to chase their own tail. These are NOT the problem. All the healers who see this know the nerf hammer is coming in 5.0. Both SCH and AST are too powerful due to their overall utility. They don't need to nerf heals or damage at all. They simply need to spread out the utility those two healers bring.
Damage is a great way to give each job it's own flavor. Think about it, how many DPS jobs do we have now? And it is the easiest role to add jobs to, so how a healer uses offensive abilities should be the least of the devs worries and ours. They can be totally creative here without too much worry of an unbalance so long as they keep it in check.
So what does this mean for an eventual 4th healer. All it means is they need to bring a utility that the other three don't have.