It seems kinda confusing to figure out which state you should sign up under for the exam.
Not sure if I should sign up under my home state (Texas) in which I hope to practice in or the state in which I'm doing my TY (PA)?

Neither, I don't think require any post graduate training to sign up.

Some states have training requirements before you can take Step 3. Sign up for any state which doesn't require you to have completed PGY 1. It doesn't matter which state you sign up under. You'll ultimately apply for a medical license where you do your residency. Most programs will require you to get a state medical license and most states will need you to take step 3 before you get your license.