Yes. You will need to prepare amended returns for each of the three years. You will do this even if your contributions were pre-tax because the IRS requires them to be reported. If your contributions were post-tax, it will lead to larger refunds (both Federal and State), so you most certainly would want to amend the returns. Here is an FAQ with links that will help you to do so.
https://ttlc.intuit.com/replies/3288565
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