Deductions & credits

You do not lose the remaining credit.  If you look at Schedule SC and the worksheet, you will see on Part 4, the Unused Septic Credit Carryover, that if you enter $3,000 in 2022, you will have $1,500 in column B as being used this year and then column C will have $1,500 as the unused credit available.  This appear on the line for 2022 and then 2024, so it is being used and carried over properly. 

If you change that number to $4,500 you will have $1,500 in column b for the portion used this year and $3,000 in column C for the unused credit available. Column C for 2024 is the amount you will carryover to 2025. (I know your credit is $3,000, but just showing how the numbers change and work)

  When you do your 2025 taxes, you would enter $1,500 in 2022 as that would be the amount you had left over although, TurboTax should import it in that column when you transfer your prior year return. 

Basically, you need to enter the amount in 2022 so it is properly used and carried over by 2022 rules.  If you enter it in 2023, it will not be treated properly when you enter it as a 2022 credit.  But also, when you look at the worksheet, the number is showing as coming from the 2023 Schedule C as a unused credit.  For 2025, you will see column a say 2024 Schedule SC line 14 column c.