I received a 3rd w2 I didn't know I would have, now I have to amend. I know to send a check, the 1040x, and what else?
You also have to include copy of the missing W2 you received. Make sure to sign the 1040X before mailing.
See the link below on where to mail your amendment.
https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1040x