You are both right but beware cos the small print says the cheapest counts as the third!
I dont know if Tescoes are taking this literally but last night I split my cat food so all felix came in one payment and gourmet came in a second cos was worried that the gourmet would be given free against the more expensive Felix.
The teller asked would I like it all to be charged together although I had seperated by dividers and I said no.