I'll just add my two cents to the whole Cure III vs everything else debate and comment on the fact what heal to use is up to preference and situation.

For example:

Decent sized damage on the two DPS but long lag between AoE attacks? Regen both DPS - Two GCDs / less MP than Cure III
Group took a huge hit, needs some fast attention and two melee DPS close by? Cure III - One GCD / Good MP to HP restored ratio
Light raid wide damage (say around 1.5K)? Medica - One GCD / More range and less overheal
etc etc

My thoughts behind the whole thing:

Cure III vs Cure II
I agree with Spoekes, Cure III on two targets is generally slightly less MP AND slightly less HP rolled up in a single GCD. Obviously, once you're in the 3+ valid target range, Cure III > Cure II (though these situations tend to be rare outside of specific scenarios)

Cure III vs Regen
On a two target scenario that's listed above, Regen will most likely prove to be the most efficient HP/MP ratio at the cost of more GCDs. Regen will provide better HP/MP ratio within 15 seconds of the initial two casts and generally always a good option as most arena wide attacks tend to take longer than 15 seconds between casts - this option is good for those looking to conserve MP but not go for high GCD efficiency.

Cure III vs Medica
Depends on the HP lose. While Medica does provide less HP it can overall be more MP efficient and generate less over heal (threat) in a single GCD. This judgement is dependent on the power of the arena sized attack and the composition of your party (you're going to have a hard time hitting ranged DPS with Cure III).

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I use a lot of comparisons because I feel it's important for WHMs to understand their options and not get pushed into a specific answer for what can be several differing scenarios. The WHM toolkit relies on standing still and casting so a good WHM will need to understand how to make the most use out of each GCD to maximize their efficiency while matching a player's play style and minimizing overheal/threat/MP loss.