My divorce was final in 2011, with an agreement that I would claim one child and their father would claim two going forward until they were no longer eligible dependents. I understood that to mean I could only claim one child for EIC purposes as well, but now I believe I was incorrect.

I am about to file my 2014 federal taxes with one dependent and three children for EIC. We have joint physical and legal custody on a 50/50 basis and their father is not eligible for EIC. The IRS walk-thru seems to substantiate my path is correct. They do not live with me LESS than half of the year. If only one of us is considered the custodial parent, I am despite the 50/50. All this being the case, I can and should go back and amend my 2011, 2012 and 2013 returns for what would be a substantial windfall...right? I was financially eligible (I believe, for all of those years.)

I used TaxAct all of those years and can access all the returns easily. A human preparer would probably have advised me on this situation better.

You have up to three years from the original filing deadline to amend your return and receive any additional refund. You can amend your past returns. The additional refunds for 2006 and 2007 are lost. you can amend 2011 return by 4/15/2015