You can gain as much length as you want. It's all about your time, dedication, and discipline. Can't gain anything if you're just playing around. Our brothers here push hard daily to get their gains. I came from 5in, currently at 7in, and aiming for 9in.
So, can you get 3in more in length? Yes. Can you go even further? Yes. Can you keep going after you reach your objective length and girth? Definitely. PE is a life long journey.