The A3 E-tron does not have the typical regeneration "levels" like you see on the VW E-golf or other EVs. It takes a little getting used to.
1. While in Auto and Drive mode, the car will coast and not regen at all. This is great for overall efficiency.
2. For maximum regen, use the shifter to put the car into sport mode, you'll have to touch the brake pedal ever so slightly to get the car to max regen, all while not engaging the friction brakes.
3. Pushing the shifter to the right for manual mode doesn't regen as aggressively as Sport mode. I also did not play with this mode to figure out the relationship of speed/gear to the amount of regeneration I could observe. With our relatively tiny HV battery (when compared to a pure EV) I throw it into S mode on long downhill roads to push as much and as fast into the battery as possible. It feels like anything other than max regen doesn't help gain back usable energy.
Be mindful of your what your brake lights are indicating as you are trying to eek in a few more miles into the battery! I inadvertantly made a guy behind me mad as it looked like I was riding my brakes all the way down a mountain road in S mode.
I have a 2018 e-tron so there is no charge mode on my car. I made myself super sick by trying to charge the battery in Sport by getting on the gas to build speed and pulling off the pedal to push the energy in hold mode back in. I looked like a moron with the yo-yo speed ups and slow downs and I think I managed to push in 3 miles of range. Not worth it.