Of those three, the MiG21 is the simplest. The F-104 and Mirage III are approximately equally as complicated (with perhaps the F-104 topping the two). The F-104 required fairly extensive training to fly properly as the Luftwaffe discovered much to their disgust - but in part that was because they changed it from a light interceptor for high level intercepts to a low-level strike aircraft.
The Mirage III was easily manufactured as Australia, South Africa and Israel discovered. It lacked a sophisticated radar though and it's missiles were, well, "adequate" just describes it. It had two reliable guns (DEFA) and was quite maneuverable, particularly when compared to the F-104. It's engine was relatively simple but lack much room for improvement, hence the creation by the Israelis of the Dagger/Nesher/Kfir series.
Out of the three, I'd say the Mirage was the best choice. It could be adapted to be a fighter-bomber and was adequate as a fighter.
Perhaps you should look at the Saab Draken as well? Roughly comparable but a much heavier armament and better engine. A twin engined development would be interesting.
Then we start getting into heavier aircraft - the F-101/F-4/F-106/Su-11/Su-15 style of aircraft? Of those, I'd place the F-4 first and foremost, followed by the F-106 and then the F-101/Su-11/Su-15.
Of course, you might like to go with something you've designed yourself?
