Definitely 2 sets, 3 if you have the time and inclination. If you don't get results with 2 sets then you may need to do 3.
Esp. during the first week of 5s you should be able to manage 3 sets. You are probably using loads not too dissimilar to those you used at the end of 10s when you were doing 2 sets, or 20 reps. So you are still doing less work.
For the second week of 5s it will be tougher to get 3 sets. That's why a lot of guys think in terms of total reps. If you got 5 reps, then 4 reps, then 3 reps, then another 3 reps with your 5RM load you would have performed 15 total reps. That's unlikely to be possible if you tried 3 x 5 reps. This way you get the work done but lessen the fatigue.
One other thing you could try for your big compounds, is to reduce the load to your 15RM after your second set of 5s and do a set of 10 reps with that. This is to keep the total work done as high as it was during the 10s without impacting on recovery much. You should find that you can get the reps with really good form.
Anyway, see how you get on with two sets first. There's no point in doing more than you need to but be prepared to do more if you still don't get results with the 5s.