You should aim to get strong letters of recommendation from attendings who know you well. A letter from an attending with whom you worked on a research project sounds like a good idea . . . did you do any clinical work with this attending as well?
Most programs expect a total of 3 letters (I think ERAS allows up to 4). I'm not sure if a letter from a radiologist is required, but I had 2 letters from IM attendings and 1 from Radiology.
Best of luck with your application and interviews!