Did you receive three (3) Form 1098s?
Company A - Enter $1 as ending mortgage principal balance for Company A - you didn't owe them anything at the end of 2020, but the program won't accept "0" as an ending balance (see below).
Company B - The transfer to Company C was not a refinance; it was just a change in who was servicing the loan. Enter $1 as ending mortgage principal balance for Company B - you didn't owe them anything at the end of 2020, but the program won't accept "0" as an ending balance (see below).
Company C - This is the loan you had at the end of the year and should accurately report your year-end mortgage principal balance. Enter that ending mortgage principal balance in Box 2 of the 1098 interview.
If you no longer have a particular mortgage (refinanced or paid off or transferred to a new lender), entering "$1" as the mortgage loan balance will not have any effect on your tax calculation and will not generate an error in either TurboTax or with the IRS,
There are many places in TurboTax where you must make a non-zero entry, primarily to let the program know that you answered the question and that the answer makes some sense.
In this case, entering "$1" tells the program that you didn't forget to answer the question and that you had at least some amount of a mortgage loan balance to justify claiming an interest
