Three days isn't a very long time. Moving can be fairly stressful and they could go off feed for a bit. Could also be in shed (are its eyes cloudy?).

The best thing you can do is just give so minnows every few days.

What are the temperatures in the enclosure? You should probably read the general garter care sheet here and you will have a much better understanding of what is going on.

How big is the snake?

Someone is gonna ask for you to send pictures, so you might as well do so now.