I think 3% is an exaggeration. And in fact, you can get 2% cash back. I don't think there's very much being charged when all is said and done.

This seems to show the fees can be considerably less than 3%, although PayPal or Square will charge nearly that:

https://www.fool.com/the-ascent/research/average-credit-card...

https://www.mastercard.us/en-us/business/overview/support/me...