As long as they are roughly the same size and sex, there is no problem housing different species together. The only species of garter that shouldn't be housed with other snakes is the Wandering garter (Thamnophis elegans vagrans) as they will cannibalize. The 30 gallon will be fine for three young ones, but as adults they will need more space. Are you getting males or females? Females get larger than males, so you would eventually need slightly more space than with three males.

Looking forward to seeing pictures! Those are some of the best looking species