The basic rule of WHMing is: the less you can cure (and the lower potency heals you can use) while still keeping everyone alive the better. Medica has 300 potency and Cure 3 has 550 potency. If Medica is enough to heal your party full (combined with heals of your healing partner), you shouldn't use Cure 3.

Cure 3 does have its uses though. It's great for situations where you have to heal a group from big damage hit (and that group is able to stack tightly) or if you have to heal a group of people who are far away from you. I generally use it for turn 5 fireballs and Garuda Extreme when I'm in different tornado place than the people I'm healing. I also use it in Ifrit Extreme after Hellfires, for example.