That's exactly what they did for the first five years. In fact, for the first two years, they drew for every lottery spot, not just for the first three. That's how the Knicks won the Ewing lottery and the Warriors, who had the worst record, finished 7th out of 7. They didn't change to a weighted system until 1990, then changed to the current system after the Magic won their second straight lottery in 1993, despite a 1 in 66 chance of doing so.