There is a gene that causes extra toes in chickens. It is found in some breeds (Silkies, Faverolles, Dorkings, Houdans, Sultans, and probably a few others that I forget.)
If they are supposed to be Barred Rock chicks, extra toes are a problem. The Plymouth Rock breed is not supposed to have extra toes, regardless of whether they are the Barred variety or any other color.
The extra toes shouldn't be a big problem for the chicks, just an indicator that they are not what you thought you were getting.
I'd be curious to see a picture of the whole chick, to see how much it looks like a Barred Rock in other respects.
I wonder if the seller mixed up the eggs, or if a rooster got in the wrong pen.
Or, as
@JacinLarkwell says, the extra toes could just pop up randomly without the chick being directly related to any chicken breed with extra toes. But since you have extra toes on two out of three chicks so far, I think it's more likely that the chicks do not actually come from two Barred Plymouth Rock parents.