It is extremely unlikely that anybody would order a shirt in three sizes. After all, each of us is currently just one size. Such grammatical speculations have little to do with real life or real usage.
It doesn't matter how many you're ordering. They still come "in" specific colours and "in" specific sizes.
I'm ordering three hundred shirts. I'm ordering one hundred in blue, in small. I'm ordering one hundred in green, in medium. I'm ordering one hundred in red, in extra large.