Rex Rosemary for Hedge

I'd like to use Rex rosemary to create a hedge. How far apart should I plant them, and would I be able to keep the hedge to 3 feet high?

If you live in zone 8 to 10, then yes Rex rosemary would be a great plant for a hedge. In any zones colder or warmer than this, most rosemaries would either perish in the winter or summer if kept out of doors. Arp rosemary will even survive zone 7 winters out of doors, without protection.
If your climatic zone is suitable, plant the plantlets about a foot (30cm) apart. If you don't mind weeding and waiting for the hedge to close for a couple of years, then you can plant them 2 feet (60cm) apart.
You should have no trouble keeping the hedge to 3 feet (one meter) in height. Just be sure to trim the hedge frequently if you have the kind of soil it loves and grows like topsy! Otherwise the top will get coarse and will need more drastic pruning to get it back into hedge shape. But you have the same problem with any other woody plant used as a hedge.