Forgot to mention that even with 3 sessions we managed to get completed in daylight. The last round would only really have to have pairs on a lane as you wouldn't expect more than 30 shooters in it. If you have an odd number then have one of the course marshals go round with the odd shooter to stop having a bottle neck of 3 shooters on a lane.
Booking in for any session should be done a pair on lane 1 (from different clubs) then on lane 3, 5, 7 etc.
Once lane 15 has 2 shooters start on lane 2, 4, 6 etc.
Once you have pairs on all 15 lanes you go through and place a third shooter on every lane.
This way - if you have a low turnout for any round then it will complete quicker. Not much point having 3 shooters on each of the first 10 lanes and the last 5 empty