It's gone "Pass," "Pass" and it's up to you. Do the rules change in third seat? You betcha! You can open with 10 pts (some say even 9!), BUT you should only do it with a Darn Good Suit. Why? Once Pard has passed, game is remote. If the Enemy win the contract, your bid places points for them, so you better have a nice hand to provide a lead!!
What about opening four-card majors in third seat? Sure. Again, Pard's "Pass" protects you. The worst that can happen is he gives you a limit raise when holding 10-11 pts. You'll survive.
Let's practice and see how this works. And talk all this over with your favorite partners.