Gohan didn't. His SSJ2 would be stronger if he kept training, but that's about it. SSJ3, initially, was only learned from otherworld. Gohan and Vegeta had no clue about it.
Super Saiyan 3 is heavily implied to be a supernatural form that Goku could only reach and maintain because he was dead and didn't have actual ki. And in the case of Gotenks, he's a magical fusion and SSJ3 burns out his fusion time anyway.
Vegito