What dtrack said. It will come with some of its own specific limitations.
-You may be dividing call amongst 3 people.
-When the first year is off service they may not take call during the day or something like that - meaning you share 2 person.
-If you don't fill a spot you may be sharing 2 person call all the time.
-You won't have the cameraderie of a larger program.
-A larger program should be definition see more cases and through this theoretically see more diversity.
Of course - perhaps you have the perfect amount of cases and immediate and easy access to your attendings, program director etc. Perhaps you are tight with simply your 3 people and their family. Perhaps the program is a country club and its just what you like or perhaps its just the right of busy and you go to bed every night tired knowing you gave it your all. A lot of programs are honestly too big and have a lot of hungry mouths. Having only 3 people limits those stupid events where everyone gets done early and then has to stand in the back of the room for some final fusion that is taking too long. It also limits the number of students who will visit, usually, and I hate to say this, but students drive residents crazy.
Awhile back I was talking some friends who were doing interviews in Dallas. They were commenting on the silliness of prospective students who didn't know anything about a program and so simply said bland things.
Resident: "Tell me something you know about this program"
Student: "Well, its a big group of residents so ya'll must really get along have great cameraderie"
Resident: "Lots of programs are big - tell me something specific about THIS program".
Size is an important characteristic but nothing beats whatever up close knowledge you can get. Clerk. Visit for a day. Call. Do something.