depends on if you use the fastest one or not. Cannot have fractional doctors or fractional patients so the normal methods for average rate calculation are not appropriate. I would have to assume that one doctor is faster than the other two so did three patients in five minutes rather than two as did the others. And then there is the very real issue that individual sickness is not identical between patients. Not a very realistic scenario, is what I am saying.
However, if you want to do the calculation for fun's sake, you have 15 work minutes from doctors (3x5 minutes), divided by 7 to give average patient care minutes, or 15/7 minutes of work is needed per patient.
New scenario : 15/7 minutes per patient times 3 patients = 45/7 work minutes, divided by 4 (doctors) = 22.5/7 (45/14) minutes per doctor, or 3 minutes plus 3/14 of a minute, or roughly 12 or 13 seconds. something along those lines. Not a realistic problem though.