I do a lot of 2 rooms (haven't done 3) reservations. If this is in the US, just reserve the 3 rooms and list 2 people in each room. It is the same price whether you have 1 or 2. There is NO reason to tell them the room has only 1 person in it. They don't ask you for the names of the other guests unless you want to tell them. The reason you want to tell them there are 6 people in the 3 rooms is because it is possible that you could get free breakfasts for the other 2 rooms also. That way you could get 6 vouchers per day. Why do you need double the vouchers? Because some hotel will let you upgrade the breakfast to full buffet with an extra voucher. And some will let you use the voucher as a fixed value to order breakfast from Grab&Go shops.

I have never checked into a Hilton hotel in the US with 2 rooms and being told that only 1 room will get free breakfast. Maybe I am just lucky.

The only time I don't list 2 people in room is when I am traveling for work. I don't want the invoice to show there are 2 guests in the room then I might have to explain to work that I didn't really bring along someone else on the business trip. And work pays for breakfast anyway. Although sometime I only get 1 bottle of water.. and only 1 cookie at doubletree.