There are 9 ways total.
Legend: s=Small,M=medium, L=Large,Lt= lettuce,T=tomato,M=Mayonnaise.
1:Hamburger-S-Lt
2:Hamburger-S-T
3:Hamburger-S-M
4:Hamburger-M-Lt
5:Hamburger-M-T
6:Hamburger-M-M
7:Hamburger-L-Lt
8:Hamburger-L-T
9:Hamburger-L-M.
which equals 9 total.