As fanfare indicates, there's no time limit to amend a return when you owe taxes. As you say, the general three-year limit is for refund returns.
You'd need to amend each year affected.
And, as relates to potential audits, fanfare is right on target again. According to How far back can the IRS go to audit my return? on the IRS' IRS Audits page:
Generally, the IRS can include returns filed within the last three years in an audit. If we identify a substantial error, we may add additional years. We usually don't go back more than the last six years.
@reeldeal
