Just switched to 3 wheels (new ST) from a lot of years on 2 and think I know what you're feeling. You head into a turn, everything is going great and suddenly the bike takes off in one direction or the other. Scared the crap out of me the first (few) times it happened. Steering is hyper twitchy. Plus it's adaptive and changes depending on speed and RPMs. Wind (or the bow wave from a car) really move the bike around.
I am no expert but here are some things that have helped me get more comfortable on the bike:
1. Look through the turn. It works on 2 wheels, it works on 3. If you're looking at the nose cone, you're going there. If you're looking at the curve exit, that's where you're going.
2. Let go and use the force. Loose hands, loose shoulders, breath.
3. Just like any vehicle, brake before the curve, fast at the apex.
4. Do swervies. On a straight road, no traffic, lots of run-off, just move the bike around. This is pretty much what's in the operator's manual but on the road instead of a parking lot and higher speeds.
5. Find a twisty road with a good surface and just ride that puppy. It's big time fun and really helps get you used to the handling.
6. Body english helps. I move one butt cheek off the seat, lean forward toward the inside wheel and the difference is amazing. Run the same stretch of road with and without moving your body around.
7. If you can find some sand, play. I hit a patch doing a 360 this morning for the first time and it was interesting to feel the bike slide, then stop sliding. The traction control stuff takes a bit of getting used to. FWIW, this time of year in NH, most back roads are just getting of the frost heave season and full of sand from the winter. Running them on 2 wheels is pretty scary when you come around a corner and see sand from side to side! On 3 wheels...pffftt, Sand? We don't care 'bout no steenking sand! There's a little skitter as you go over then on to the next curve! (please note, you can and will slide. The traction control will not prevent you from going into the outer lane if you're moving too fast)
8. Try the ABS. While the 0 - 60 is impressive, the 60-0 is freaking amazing! I swear I went 50-0 in about 30'...I know that's not possible but that thing flat stops when you mash the brakes. Be careful; it stops fast enough you can toss yourself off the front if you're not paying attention.
The dealer told me when I picked up the ST that the more I rode, the more fun it would become. Moving from a 650 Ninja to the ST was a huge adjustment. Carving back roads is just as much fun but totally different. Enjoy!