As others have said, It is hit and miss, but you need to always tell them if there is 3 in the room. For most US hotels it is no problem, outside the US it may be iffy. As far as breakfast, bene's clearly state 2 per room. Almost always I am handing 3 sets of coupons (when I register 3 in the room).

Last week at a HGI the night clerk was very clear that I would have to pay for my niece's breakfast (I was traveling with my Mom, so 2 adults and 1 kid). No problem. The next morning had some of the best french toast ever, sat around waiting for the bill after I gave the waiter the 2 coupons. I finally asked for the bill for the kids breakfast, and he said, "no the coupons covered all of you".

Hope this helps.