I have a question regarding how to handle my mortgage interest deduction for last year.

My general understanding is that I can deduct the mortgage interest on two properties for personal-use. For a portion of 2017 I owned three personal-use properties and am not sure how to proceed.

Property "A" - This is a property that is occupied by a relative. I owned it all year. In past years I treated this as a second property.

Property "B" - This is the property I occupied at the beginning of 2017. It was sold in September 2017. In past years I treated this as my main property.

Property "C" - This is the property I occupied at the end of 2017. It was purchased in May 2017.

So from May to September I owned three properties. There was not a precise date that I moved from "B" to "C" as the move was rather long an drawn out, but I was completely out of "B" by the end of August.

I don't think I can deduct all the mortgage interest paid on all three properties all year long.

One way to handle it would be to pick an arbitrary date between purchase and sale, call that my "move date", calculate the interest paid on each mortgage relative to that date, and claim that fraction of the interest for the respective properties.

This is less than optimal as the interest paid on "B" and "C" is much higher than "A". So another option, if legal, would be to claim A+B up to the purchase date, B+C from purchase to sale, then A+C after the sale. But I don't know if it is allowed for me to swap the main and second properties around like that.

Additionally, no matter how I handle it the amount of mortgage interest I claim won't match the total that will be shown on the 1098 forms (since I'll only be claiming parts of each). I don't know if this is a problem.

Does anyone have advise on which track I should follow?