If you take it real easy you can hit wherever you want on the board on each throw. It takes extreme patience, but it can be done. Make a fist with your hand and aim at the board. If you slowly pull back, the reticule will lock in place. This gives you time to decide if you want to take that shot where the reticule locked. If you are unsure in any way, don’t shoot, just move your hand around to reset the reticule. You won’t be able adjust on the fly accurately either so don’t try that. You want to be absolutely sure for some of these achievements.
If you pull back and it stops exactly where you want it, now you can throw your dart. You’ll have to punch forward with some effort, but not too much, otherwise it won’t fly straight. If you go too slow the reticule will reset, as it won’t recognise a shot. That’s OK though and better than an errant shot. Concentrate on punching very straight, with just enough force for the game to recognise a throw. There's a bit of a learning curve but with patience and time you'll get the hang of it. Be prepared for some frustration though, it’s not easy by any means.
To fit three in such a small space (triple-20, bullseye), you can lean or move slightly to each side to fit when on one side, in the middle, and one on the other side. Combine this with where your aiming reticule is and you can fit all three in the bullseye or triple-20.