I forgot to file taxes for pre-ipo capital gains. My company gave stock for $1, but, when I bought it, it's Fair market value was $5 and I did it for a couple of years.

For 2021 taxes, I can do it now. But, how about 2019, and 2020?
Can I file everything together showing all the gains in 2021 itself, not to have an audit? OR Do I've to amend 2019, 2020 taxes as well? It was an honest mistake as I did not knew that I've to pay taxes for those, even if my company has not IPO yet.