Print out the Schedule 3 that shows your credit.
Get Form 1040-X from the IRS web site.
If you aren’t changing any dollar amounts you originally reported, you don't have to do column A B C.
So leave that all blank.
Just fill out Part III explanation stating that " IRS needs Schedule 3 and here it is."
Include a copy of the CP notice and allow 30-60 or more days for a resolution.
IRS will attach the Schedule to your return and mark it to be processed some more.
