Basic Principles for WHM - What and what not to do.

[KisaiTenshi]

No assumptions about tanks and range to tank. Just assuming to players got damage and they are close enough together to receive cure III.
Sometimes you can place yourself in the middle of 2 players and cast it on yourself to "bridge" the gap and heal both.

Cure III is MP hungry if used wrong, on the other hand its MP and time efficient if used right. Of course a spell that heals multiple players does cost more than single target heals, but common thats obvious.
Again, if you can hit 2+ players with it, casting 2 cure II is slightly more MP hungry than one cure III, while cure III is saving a GCD.

The only situation where Cure III is even usable IMO is if the Tank and both DPS are melee because they will both be close. The rest of the time Medica is better because it has farther reach. Cure III's reach is less than Holy's.