There is always uncertainty surrounding this. On one side are people who think you should have more recommendations from radiologists (2 radiologist and 1 surgeon, say). One the other are those who think you should have more from people you've worked closely with clinically, because they're in the best position to answer the question program directors have: "Will this person be a good resident?"
My advice? Definitely have one letter from someone you worked closely with clinically (surgeon, or internist, or whatever). And definitely have one letter from a radiologist who knows you. But make the third letter from someone who knows you very well, and can say, "I know that Horatio will be a great resident." As long as that person is an attending in something, that's the third letter you want.