IF you want a good jogging stroller, you're going to have to put out $350-400 or so minimum for a double. The ones that are much less than that - let's just say that you'll worry about your kids falling out and they probably won't last long. If you don't want to spend the extra money for the third seat, maybe you could get your 3 yr old a bike with training wheels (from Wal-mart) and let her ride while you walk/jog. Richardson BikeMart has some double strollers, several varieties. (They are bike trailers that convert into strollers.
My SIL also has a stroller she got from Target that she really likes. They are probably only online now. I think it was $160 or so. She uses it all the time and it works really well. Of course, it's only a double.
Good Luck