Turbotax 2018 Tubotax used three different amounts on line 12--$0,$3641, and $2519. How do you explain that?

Line 12 of what form? And what do mean that it used 3 different amounts on the same line? On most forms there's only room for one amount on each line. Do you mean that it had different amounts at different times? The amounts on the tax forms will change as you enter more information.

Are you talking about line 12 on Form 1040? On the 2018 Form 1040 line 12a, on the left, is the child tax credit and credit for other dependents. The total amount on line 12, on the right, is the amount on line 12a plus the total credits from Schedule 3 line 55. A change in any of the credits on line 12a or on Schedule 3 would change the amount on Form 1040 line 12.