First off, I am assuming you are talking about the fruits, not the pads, both of which are edible.

1) All fruits of the family Cactaceae are edible, prickly pear included.

2) Unripe prickly pears are pretty unpleasant tasting, very bitter. It is always best to let them get fully ripe while still on the plant though.

3) If it really was green, then no. If they have started to turn red, sometimes they will completely ripen off the plant.

I would add: Don't let the fruit ripen too much, even if it's still on the plant. I've found nasty critters inside a few.