Are you talking about away rotations? First of all, you should check your schedule to determine how much time you have to do away rotations. Next, while it's ok and common to do aways in different areas, you may want to find out what people at your school typically do, what people in the specialty(s) that you're interested in typically do, etc. I don't think it's that common to do aways in 3 different rotations and I think it would be especially weird to do 2-3 aways at the same institution in different fields, but <shrug>
I'm going into radiology and typically most people do a single away and a home rads "sub-i" I will probably do two aways more for my interest than anything else, because I have two programs I am especially interested in right now.