At 3 psi you cannot drive too far on a run-flat without losing sidewall strength. The range is dependent on speed (driving on the flat) and will be further limited if the tire went flat due to a sidewall tear or puncture. If the tire is repairable, it will have reduced range if it goes flat again.
Make sure your service tech is well-informed on these limitations.
The maximum range to drive on a run-flat is often quoted as 50 miles.
http://www.popularmechanics.com/cars...ires-17069881/