I agree with the others who think you are beating the wrong dead horse!
Also you're overlooking the fact that companies have to pay to deposit checks and cash, so even if a shop was to go cash only, they wouldn't be able to drop their prices by 3% because they would be paying more for cash deposit services (e.g. armoured carriers), higher insurance premiums because they have thousands in cash sitting on the premises which makes them a more likely target for robbers, they have to hedge against counterfeit notes, there's more risk for employee theft etc...
Yes card companies make money on each transaction, but they price themselves so it's a wash for the merchant.