You can amend up to 3 years after the filing deadline of your return or 2 years after you have paid the taxes, whichever is later.
Right now you can amend your 2015, 2016 and 2017 tax returns. If there is a refund, you will receive the refund. If there is tax due, you have to pay the taxes.
Amending your tax returns does not automatically raise a red flag with the IRS, and you will not be audited just for amending.