Ill give you the basic english explanation. I know all this talk of genotypes and odds can be confusing.

Bps can carry more than 1 gene at a time. In your explanation you mentioned 2 one gene snakes. To get a 3 gene snake you need at least one of them to be 2 gene.

Example:
Pastel mojave male
Breed with
Pinstripe female

You have a chance at getting a baby snake that has all 3 genes:
Mojave pastel pinstripe.

Each gene will have a 50/50 chance to be transferred to the baby.