I need to enter a third 1099R. TurboTax wont allow this.

Are you using the below process?

1. Open (continue) return if you don't already have it open.
2. Inside TurboTax, search for 1099-R and select the Jump to link in the search results.
3. Answer Yes on the Your 1099-R screen, then select Continue.
   • If you land on the Your 1099-R Entries screen instead, select Add Another 1099-R.
4. Select how you want to enter your 1099-R (type it in yourself) and then follow the instructions. You can add up to 20 1099-Rs.