Cure III a tad underwhelming?

Cure 3 most definitely has it's uses, just as said, that small radius limits it's practicality.

As mentioned, it is super useful for the closing stages of turn 4 to deal with emergency override (Until your DPS gets good enough that you don't actually hit that point anymore!).

As far as T5 goes, I don't really use it during conflags as it just doesn't heal enough, take into account the additional bonus that medica II's regen offers in soaking conflag damage and Cure III quickly goes on the back burner during this phase, I do however use Cure III during the later portion of the dive phase to deal with snake damage potentially being spread out.

I've had occasions where people have been packed up tightly enough during garuda's sisters to get good use of it there. I had an amusing Titan run where all DPS and the second healer were dead leaving just the tank and myself standing, cue healing just enough to keep the tank up for mountain buster then hopping under titan and using a couple of cure III's to keep us both going through a 5 stomp. Medica II into medica spam wouldn't have had the throughput to keep myself going and get the tank topped up enough to survive my LB3 animation lock.

Whilst yes, it certainly is a niche spell and certainly not one for every encounter, it is a potentially useful little addition to our toolkit providing hugely bursty multi target healing when the situation is right.