Yes, of course. Here is a simplified example with two instead of three mortgages. Lets start with the mortgage from 2016 with an average balance of $1,000,000 and interest of $20,000 for the last year. Since the limit for a pre 2017 mortgage is $1,000,000, all $20,000 of its interest is deductible.
Now, lets add a 2020 mortgage of $2,000,000 and the same interest of $20,000. This can be because a banker is my friend or the mortgage simply started later in the year, I think it does not matter. Here is the worksheet taken from TurboTax verbatim and filled with the above numbers. Notice how it calculates my deductible to be only $13,333 this time, but I would expect it to remain $20,000. And if I increase the amount of the second loan further, my deductible will go down. Also notice the instruction for line 18 from the sheet. It calls for subtracting the factor from amount which is obviously wrong.
| Part 1 - Qualified Loan Limit |
| Qualified loans acquired prior to October 14, 1987 |
| 1 | Average balance of debt acquired prior to October 14, 1987 (grandfathered debt) | |
| Qualified loans acquired after October 13, 1987 and before December 16, 2017 |
| 2 | Average balance of debt acquired after October 13, 1987 and before 12/16/17 | 1000000 |
| 3 | Enter $1,000,000 ($500,000 if married filing separately) | 1000000 |
| 4 | Enter larger of the amount on line 1 or the amount on line 3 | 1000000 |
| 5 | Add the amounts on lines 1 and 2. Enter the total here | 1000000 |
| 6 | Enter the smaller of the amount on line 4 or the amount on line 5 | 1000000 |
| Qualified loans acquired after December 15, 2017 |
| 7 | Average balance of debt acquired after December 15, 2017 | 2000000 |
| 8 | Enter $750,000 ($375,000 if married filing separately) | 750000 |
| 9 | Enter larger of the amount on line 6 and the amount on line 8 | 1000000 |
| 10 | Add the amounts on lines 6 and 7. Enter the total here. | 3000000 |
| Total qualified loans |
| 11 | Enter the smaller of line 9 or line 10. This is your qualified loan limit | 1000000 |
| Part 2 - Deductible Home Mortgage Interest |
| 12 | Enter the total of the average balances of all mortgages from lines 1, 2 and 7 on all qualified homes. See the line 12 instructions | 3000000 |
| 13 | Total amount of interest that you paid on the loans from line 12 reported on form 1098 | 40000 |
| 14 | Total amount of interest that you paid on the loans from line 12 not reported on form 1098 | |
| 15 | Divide line 11 by line 12 | 0.333 |
| 16 | Multiply line 13 by the decimal amount on line 15. Enter the result. This is deductible home mortgage interest for loans reported on form 1098 Enter this amount on Schedule A line 8a | 13333 |
| 17 | Multiply line 14 by the decimal amount on line 15. Enter the result. This is deductible home mortgage interest for loans not reported on form 1098. Enter this amount on Schedule A line 8b | 0 |
| 18 | Subtract line 15 from line 13. This is not home mortgage interest | |
