Diet makes a huge difference in training progress. It can be the difference between ending LP squatting 225x5 and 355x5.
For linear progression, it's 5x3, not 3x3, but you're going to do 3s eventually anyway. At your age, it might be a good idea to try it sooner rather than later.
You're welcome, and good luck. If you have other questions, feel free to post back here or email me if you want a more in-depth consult. It might be a good idea to visit Cameron Cox in Charleston depending on where you are in SC.