I have a question about attacking with Brother Leon. Using Full Auto, he may attack up to 3 times. If I miss on any of the three attempts, can I use a support token for 3 more shots?

One Suport Token for every re-roll, not another series of three shots. They are three separate shots resolved as ususal.

Kositp said:

No.

See page 15: "A player may spend (discard) a Support Token from one of his Space Marines when it is attacking or defending to reroll a die that was just rolled."

Thanks for the answers. It helps a lot.