80% letoff makes shooting fingers more of a challenge, but it can be done. Adjust your draw length so that you're right on the wall when you hit anchor, then you can use the wall to help the string escape smoothly off your fingertips as the natural result of applying back tension as you pull through the shot. Very important to remember that releasing is NOT an action, it is an inaction. You are holding the string on your fingers at anchor. When you want to release, you simply quit holding it as you pull through the wall.

It's easier to do than it is to explain.

By the way, it seems most people who shoot fingers on high letoff bows find it helpful to use a 2-finger release than a 3-finger release. Most drop the index finger off the string when they get the bow drawn. The rest drop the ring finger.