How do I enter tuition reimbursement reported in box 3 of 1099-misc to be reported on line 21 on form 1040?

That depends on who reimbursed you.  If it was your employer, it should have been added to you W-2 and not reported on a 1099-Misc  The income is subject to FICA tax, as well as income tax.  

What used to be line 21 of form 1040 (before 2018) is now line 8 of Schedule 1, which ends up on line 8 of form 1040. 

TurboTax (TT) aggressively steers you to treating it as  self employment income. 

 That said, the key to getting TT to treat it as other income  (line 8 of Sch 1) is to follow these steps:

Enter the 1099-Misc

On the next screen Describe what the payment was for

On the next screen select "none of these apply"

On the next screen select "No it didn't involve work like my main job"

On the next screen select "I got it in 2020" ONLY

On the next screen select No, it didn't involve intent to earn money