They were mating. The large one was a female and the smaller ones were males. Most garter snake species exhibit sexual dimorphism in size. Usually a single female is courted by several males. Usually garters mate in the Spring but if conditions are right and the hormones are flowing then they will mate in the fall as well.
I hope that helps.
>>Hello. I write for a newspaper here in PA. The other day someone stopped at my house and asked me to come take a pix for them. It was three garter snakes, one much larger than the other two, on TOP of a shrub all wrapped around together. I took a pixcture of this but need to identify just what it was for the paper, please help-----I plan on giving this sight credit for the info, so please be sure not to be wrong-lol.
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from west of the continental divide.