We have traveled extensively with Hilton, booked as 2 adults and then brought 1 or 2 children without problem as long as they are in existing bedding.

From my experience, If the room states a maximum of 3, do not worry, even if its booked as 2, especially as your child is only 3. We have often needed to leave out one or both of the children in the booking, especially when outside the USA. When you search for 2 adults plus children on the hilton site, it seems the default is for children in their own bedding but often in description it will mention children free in existing bedding. If it isn't in the description, it can often be found under family policy on the hotel's website. In the additional comments you can mention that you have a 3 year old in existing bedding.