*Let’s say I contributed $5,000 into a non-deductible IRA each of the past 3 years.*

Presumably, you filed a Form 8606 to document the non-deductible contribution for each of those 3 years. If not, now would be the time to do so.

*This is the only IRA I have.*

This is important. The answers would be different if you have any T-IRAs with pre-tax money, including contributions, rollovers and/or earnings.

*So my basis is $15,000. Right now, the value of the IRA is $12,000. If I convert this IRA to a ROTH IRA, what happens to the $3,000 of basis not used?*

Before the TCJA, you would have been able to deduct this loss as a miscellaneous deduction by closing out the IRA through the Roth conversion. As with all miscellaneous deductions, it would have been subject to the 2% of your AGI floor (so your AGI would have to be less than $150k for a $3k loss) and your total Schedule A deductions would need to be more than your standard deduction in order for you to get a tax benefit.

However, since the TCJA did away with miscellaneous deductions (at least until 2026), you don’t get the benefit of a miscellaneous deduction from the loss by closing out your IRA.

*If I put a new $5,000 into a non-deductible IRA this year, can I say I have a $8,000 basis?*

I will suggest that there would be a better trail if you make the 2022 non-deductible contribution before you do any conversion to Roth accounts during 2022. But in going through the Form 8606 https://www.irs.gov/pub/irs-pdf/f8606.pdf the amounts asked for are on an annual basis. So my take on it is that by making your 2022 contribution, you would preserve the $3,000 basis from the prior years, giving you an $8,000 basis in your non-deductible IRA.

AJ