Hi You are in a 3 bed place, the odd room is a garage.
You sound like you are in a rented property.
housing benefit only pay a certain maximum amount for each property.
Housing benefit are usually helpful and if you phone them they will tell you how many bedrooms you are entitled to IE a two or three bedroom property, this will depend on the age of your three children.
Benefits have a cap on the amounts paid for rent and each area is different, so you need to know if there is going to be a shortfall in your monthly payment from them as you will have to top that up from the small amount you have left to live on !
Another essential tip is never ever be late for appointments they make for you to get back to work as I heard even 5 mins late they can stop your benefit for up to 6 weeks so dont ever risk it. If you have little ones under 5 I dont think you have this problem.