Yes, you can list the home as a three (3) bedroom unit even though you must pass through the master bedroom to get to the third bedroom which is in the attic assuming the attic are is actually used as a bedroom.In order to have no confusion, you probably need to state that attic space is used as a bedroom but that there is no independent access to it unless one goes through another bedroom.In California, listing agents need to be careful about what is stated in the multiple service listing (mls) regarding a piece of property in that any misinformation in the mls statement possibly could result in "strict liability" for any damages proven if the information is proven to be incorrect and the buyer is damaged.