Can garter snakes of different size be housed together?

Some subspecies of Garter Snake are more prone to cannibalism than others.

Plains GS (Thamnophis radix) and Western Terrestrial GS (Thamnophis elegans) are common in the area where I live. The latter are more prone to cannibalism, while we've never experienced this problem with the Plains. In the past I had placed different sizes/ages of Thamnophis radix together without mishap. I'm not familiar with the Eastern GS, sorry. You can look online to see if yours is prone to cannibalistic behavior.

I'm not too sure about that tank housing 3 GS's. Although, personally, I don't recommend housing multiples of ANY number/snake species together permanently.

Also, this is kind of a big deal.... Consider the breeding opportunists between these unprobed individuals. Are you prepared for a potentially gravid female? Do you have enough space/resources for hatchlings? I suppose you could simply set and potential offspring "free". I for one, however, would feel obligated do start them off at least.

Take care.

Not a good idea. The tank is already belonging to another snake and putting in a stranger will cause a fight in which all of the snakes die. Plus you should not be housing the other two together anyways.