Getting a simple house mortgage with 3 people ?

So i'm looking to buy a house within the Central Jersey area and i was wondering if i could put a third person on the loan. It would be me and my wife then added onto it will be her mother ( my mother in law ) since she will live within that property ? I'm asking because somebody recently told me when there is more then two people on a loan it becomes o investment property ? any feed back would be great!