Is it a rule of thumb to only add one at a time or can you simply add 2 or 3 at a time?
The size of colony says how much you may add room.
I have here flow too, but however nights and some days are cold.
What I do, I give now one box under the brood box and one box over the brood.
if bees do not fill all gaps in the hive, I give
only under the brood.
to add 3, you may do it but you spoil the hive build up.
When hive has 6 boxes allready, you may add 3 more, but it is not wise.
I do not use excluder.