You can use each type of casting actions as many times as you have actions to spend on it. So for example, you could use the 1-Action version of Heal three times in one turn, if you wanted to do a lot of healing to just one ally (or a lot of damage to just one undead). I only know this because in the comments of the spell blog post, someone asked about the wording on Heal’s Heighten effect (+1d8 or +2d8 if you are using the one or two action version to heal the living) and if it was intended that using Heal to damage undead scaled so poorly, Siefter said it was because they have to be very careful with one-Action spells that do damage, since they can potentially be used up to three times in one turn.