How do I enter a 1099R that will not be sent to me?

You can contact the bank/broker for the 3rd IRA to get the information. You can have multiple accounts, but each sends you a form with your RMD near the beginning of each year so you can take the appropriate funds during the year.  It could be they posted it online rather than mailing it to you.

You can calculate it from the IRA balance 12/31/2023 and the age-dependent divisor (Google for it).