If the prepaid interest is not included in the 1098s, you can enter the 1098 and check the box for "the amount of interest I am claiming is different than on the 1098". Enter the total amount (in other words, for lender A, add the amount prepaid to lender A to the regular interest included in the 1098; do the same separately for lender B, etc.) You will be given a space to write an explanation. Keep it simple. Keep your proof in case the IRS sends a letter asking for more information.