Technically speaking, if you did not include (foreign) taxable interest on your 2014 and 2015 tax returns, then yes, you should now amend them to do so. Of course, as a practical matter, you are free to do whatever you like.
For more information on how to amend tax returns, for the 2014 and 2015 tax years, please visit the following webpages:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
https://turbotax.intuit.com/tax-tools/tax-tips/Taxes-101/How-to-File-an-Amended-Return-With-the-IRS/...
Please don't forget any state tax return(s) if they are relevant to your situation as well.
With respect to FBAR requirements, otherwise known as FinCen Form 114, you can file prior year disclosures, as well as current year disclosures, all through the same US Treasury Department website here:
https://www.fincen.gov/report-foreign-bank-and-financial-accounts
While each year requires its own form, it is the same webpage that is currently used for all years. Furthermore, FinCen Form 114 reporting is completely separated from tax return reporting (and IRS Form 8938, foreign financial account disclosure required by FATCA laws). Thus, you don't necessarily need to file Form 1040X amended returns before filing prior-year FinCen Form 114 disclosures . . . or do the reverse.
In other words, now that you know you need to file these various documents, you may complete them in any order you like. Once caveat, though, applies. When you file multiple Form 1040X amended tax returns, just make sure that each tax year is sent in its own separate envelope. That keeps the revenue authorities from becoming confused, and unduly delaying the processing of your tax documents.
Finally, for cases where your foreign interest (or dividend) income is not reported to you on a traditional (US) Form 1099-INT or 1099-DIV, then please refer to this AnswerXchange webpage for further instructions:
https://ttlc.intuit.com/questions/3988314-i-did-not-receive-int-1099-how-to-insert-data-in-tax-retur...Thank you for asking this important question.
