Canadian perspective
There are some instances where the 3NM radar separation might not need to be applied. If a heading off the airport that lies close to the approach to the other airport can be proven (throught a flight check or other esoterical method) to be geographically clear, then it could be done.
Procedures would have to be established, I suppose, providing for the arrival to be fully established prior to a certain point on final or prior to releasing the succeeding aircraft. The type of approach in use (precision or not) would have to be considered as well.
SAO