Question
1.  The leaves are turning Yellow and dropping.  It seems too earlier for this here in eastern Tennessee.

2. How to prune my knockout roses to keep them 2-3'?

Answer
Roses drop their leaves for a couple of reasons. One is that they are not getting enough water and the other is because they are shutting down for the winter. If you have had any extra warm weather this summer, then it is not unusual for roses to shut down earlier as the heat puts them under stress and they don't act normal. Yellow leaves at this time of year really isn't anything to worry about. Yellow leaves in the spring is not a good sign and I would be concerned. If your roses grew and flowered well over the summer, then they are doing fine. Keeping a rose bush down below what it wants ti grow is hard to do.  In your area, Knock Out roses can get really tall. I would prune them right back in the early spring to 18 inches tall. Then if they throw up extra tall canes in the summer, I would cut those canes back even with the other canes in the bush. Pruning in the spring activates growth but  pruning in the summer doesn't activate them as much.