I would be filing amendment returns and paying social security taxes for these years in order to gain my work credits for being self employed those years. I was not aware of the optional method until this year. And other sources are saying you can go back 3 years so does that mean I cannot file an amendment for 2014 even though I am paying taxes and not getting a refund?
You may file an amended return from before 2015; however, it is likely that the IRS will not accept payment even if they change the account. You would need to contact Social Security on how that would affect your case. Regardless, 2014 can no longer be filed through TurboTax.
However, if you needed to amend the 2015 and 2016 returns you may do so. You will want to mail in the 2015 return before April 15 to guarantee that the amendment adjusts the return before the statute of limitations has expired if it is necessary that the payment for the change also be credited to your account.
The following FAQ provides instructions on how to prepare amended returns for 2015, 2016 (and 2017 and 2018 as well). Click the embedded link for the year you need to amend: https://ttlc.intuit.com/replies/3288565