1. WHM and SCH. Most serious raid groups will try to get one of each.

2. AST. Any of the 3 healers can clear PUG content just fine. No one should say anything about the types of healers that show up in DF. If we're talking serious business end-game raiding (Savage), then you'll have to coordinate with the other healer because no one wants to double up (WHM/WHM, SCH/SCH, AST/AST). The game as it stands heavily favours a WHM/SCH combination because they synergise better than any combination involving an AST. That's not to say that groups wont take an AST, or that ASTs havent cleared savage, but you will have a more difficult time finding a group and the learning curve will be steeper.

3. SCH > AST > WHM. In my experience.