I've just learned that I have (significant!) medical expenses from past years (2018, 2020) that qualify as deductions. Can I claim them? Do I file each year separately?

My taxes have been filed by professionals for the last several years, so everything else is all squared away. I was just not made aware that these things qualified as medical expenses. Can I add them to this year's deductions? Or is some accounting wizardry required to retroactively add them into the year they happened? Or did I just miss the boat and I cannot claim them at all? I still have official documentation and receipts and whatnot, if that helps. If anyone has any advice for backfiling previous deductions it would be most welcome! Thanks!