I made my last three payments on my federal 2015 installment plan 100.00 each in 2018 and I am trying to figure out where to enter that, it was entered in prior years?

[JohnW15]

If you're saying that you owed federal taxes for a prior tax year (2015), and paid off some of those taxes in 2018, those taxes aren't deductible on your 2018 federal return. 

Unfortunately, federal taxes paid for any tax year can't be deducted on a federal return. 

If you've deducted federal taxes for 2015, but paid in a later year, on a tax return for that later year, you should amend the returns for those later years to remove these deductions.

Please see the TurboTax FAQ "How to amend (change or correct) a return you already filed," below, for instruction on how to amend your previously-filed tax return.

https://ttlc.intuit.com/replies/3288565

 If your state has a deduction for federal taxes paid, that will be covered in TurboTax's state interview.