following is from irs tax tip  2018-118

Taxpayers should also not amend if they forgot to include a required form or schedule. The IRS will mail a request about the missing item.

I think you should just include it in 2020 and I doubt the irs will catch the missing years. I would also fill out the form 8997 for missing years and put in clients file, so you can send it in if IRS ever asks for it