How is it worth for the bank to give 3% mortgage over long term?

If I grab my minimal math skills, I feel like the bank is just losing money. How, and when do banks give 3% mortgages? What is the math behind it?

submitted by /u/RedditsXooro
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source https://www.reddit.com/r/RealEstate/comments/1n1cflo/how_is_it_worth_for_the_bank_to_give_3_mortgage/