I received a RRRB-1099. It shows $21204 received from RRRB in Box 3. Where should that be entered on the IRS form?

An RRB-1099 is entered in the Social Security section of the program.  You only enter the box 5 amount for the benefits received.  Make sure that you check the box Railroad Retirement Benefits (Form RRB-1099)

To enter Social Security benefits reported on form SSA-1099/RRB-1099

• Click on Federal Taxes (Personal using Home and Business)
• Click on Wages and Income (Personal Income using Home and Business)
• Click on I'll choose what I work on (if shown)
• Scroll down to Retirement Plans and Social Security
• On Social Security (SSA-1099, RRB-1099), click the start or update button

Or enter social security benefits in the Search box located in the upper right of the program screen.  Click on Jump to social security benefits

Or are you referring to a RRB-1099-R for pension income where box 3 is the Employee Contributions.  If so, then follow this procedure -

To enter a form 1099-R -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Wages and Income (Personal Income using Home and Business)
• Click on I'll choose what I work on (if shown)
• Scroll down to Retirement Plans and Social Security
• On IRA, 401(k), Pension Plan Withdrawals (1099-R), click the start or update button

Or enter 1099 r in the Search box located in the upper right of the program screen.  Click on Jump to 1099 r

Make sure that you select the RRB-1099-R from the list of form selections.