Difficult but nice upgrade
I decided to upgrade to 3H because my rear air struts had a problem where one side would always be higher than the other at the same PSI, and it was driving me crazy. Installation was rather difficult because there's NO videos on youtube or decent directions online on where to install the sensors on a 15-21 WRX/STI, so I spent a good amount of trial/error creating my own metal brackets and all that.
Once installed, I was having some problems with calibration - when re-calibrating from 3P to 3H, I kept getting errors, and it seemed like the sensors weren't talking to the manifold. After reaching out to Airlift Performance, I got it figured out by doing a factory reset to the system, and then doing a fresh calibration.
Overall, pretty happy with 3H - that height difference in my rear is gone, and it's nice when somebody gets in the back of my car, and I don't need to worry about fixing my ride height - it does it all automatically.