Need Pinewood Derby mathhelp to design software?

I would like to design a program to use in a pinewood derby race so that all racers will race everyone at least once and no more than once per round. I have a track with 3 lanes and any number of racers from 15 to 50. I would like the software to figure out how many dummy racers we need and add them so that each race will only race once per round and there will be as many rounds as I need so that each racer races each other at least one time. Also, the racers need to switch lanes so there is no advantage to racing in any one particular lane. Any Suggestions?