Mortgage broker charging 3% fee

Hi, we are first home buyers and our mortgage broker just suddenly required us that we pay them 3% of the loan amount. They never told us about this fee in the beginning. They said they were working with a private lender at first but decided to go with a bank as this will give us a much lower rate. They said we are approved by an A lender and that we are scheduled to sign the contract next week. Is this a reasonable fee for us to pay? Is this even allowed? The sad part us is they made us sign this paper that we agree we will pay them 3%. Can we still back out?