RE: 1.  If they elected in 2020 to spread it out over 3 yrs,

2.  but want to include the total amount on the 2021 tax return,

3.  do you put the total amount instead of 1/3 on line 12 and leave line 15 with 1/3 amount?

I must confess I did NOT read the earlier posts.  I may be barking up the wrong tree in part of my comments, but in the interest of address 1 and 2, AND in the interest of barking up the wrong tree (BUTWT)  here it is:

1.  Folks keep saying "elect in 2020 to spread it out over three years."  Taxpayers do NOT elect to spread over 3 years.  It is the default.  Taxpayer MAY ELECT to claim all qualified distributions (as defined in the code) in ONE years.  READ THE CODE and IRS Regulations and the instructions of 2020 Form 8915E.  

I do not know if there is a time limit for the election.  This post seems to talk about filing 2020 late (possibly BUTWT here).

2.  I'm scratching my head, bemused by the logic.  (Possibly BUTWT here).  IF someone "elected in 2020 to spread it all in three years" , how could that someone "want to include the total amount on the 2021 tax return"?  Another case of Delorean at 88 mph??? (I spelled Delorean this way to troll Bob to come in and correct me: It's DMC DeLorean!!  But, hahaha, he can't do all caps here!!!)

3.  If I type something here, it'd definitely be BUTWT.

@IRonMaN : Perhaps you should change your signature line to:

I come here to stand with Ukraine and to see Joshua barking up the wrong tree....

Have to go to, some other trees are worth barking up....