House hacking question

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Hello. I am in the process of buying my first home and am planning on renting it out. I am looking at duplex’s but found a 4 bedroom single family home that I like. My question is that if I bought this house and rented out the other 3 rooms to my friends, would I be able to use their rent as rental income to improve my dti to get a 2nd mortgage eventually? Or would their rent not count towards that. Also, after we eventually moved out and I rented the place out to tenants, how long would they have to live there for me to use their income to help my dti? Thanks.