First, take all of the missed RMD's ASAP so you can say in the explanation statement that the RMD has been taken as soon as the oversight was found.
You need a separate 5329 for each missed year. The 5329 must be for that year (see the link below for past years forms).
From 5329 instructions:Waiver of tax. The IRS can waive part or all of this tax if you can show that any shortfall in the amount of distributions was due to reasonable error and you are taking reasonable steps to remedy the shortfall. If you believe you qualify for this relief, attach a statement of explanation and file Form 5329 as follows.
1. Complete lines 52 and 53 as instructed.
2. Enter “RC” and the amount you want waived in parentheses on the dotted line next to line 54. Subtract this amount from the total shortfall you figured without regard to the waiver, and enter the result on line 54.
3. Complete line 55 as instructed. You must pay any tax due that is reported on line 55.
The IRS will review the information you provide and decide whether to grant your request for a waiver.
Past years forms here: