Hi, my name is XXXXX XXXXX X will be helping you today.There is no simple formula because it really depends on the usage of each person, which can vary drastically, especially if one stays home all the time and one works all day.Your best bet is to determine if the current 3 people use around the same amount of utilities, meaning they all spend about the same amount of time in the house and use the same things. If one of the 3 is a baby, obviously you would only base any calculations on 2 people, as an example.
I would just figure out your total utility cost and divide that by 3 (if that is the amount of people that are closely using the same utilities.) That number will give you a utility cost per person and that would also be an approximation of what an additional person would cost. If you are satisfied with this answer please leave me a positive rating at your convenience.