The three-action unheightened Heal spell heals 1d8. The three casting options are mutually exclusive options which do not interact with each other.
The spell says, “The number of actions you spend when casting this spell determines …”. If you spend 3 actions, you use the properties from the 3-action entry.
I don’t have a hard proof, but inheriting properties from the lower-action versions creates a lot of contradictions. For example, if you used the system to cast a 3-action Heal spell and inherited the properties from the 1-action and 2-action versions it would somehow simultaneously need to have a range of touch, 30 feet, and a 30 foot emanation.