Something that always works for me is thawing one rat with two mice in a plastic bag, slowly, because it will often make the mice bleed from the nose/mouth, and then rub the mouse blood on the rat. Another trick, after you've thawed the mice and rat, is using a blowdryer, pre-scent with a thawed mouse very close to the cage. This drives them nuts, and by the time they realize that it's a rat you're giving them, it's generally already in their mouth, and they realize that they can eat it.