Yes, you can apply a carryforward of your capital losses by amending prior years' returns. You may file an amended return up to three years after the original due date. That means you missed out on 2017 and have about a week left to get a postmark on your 2018 amended return, assuming you originally filed them on time.
Since you missed out on 2017, you still need to calculate how much of your loss would have been used in 2017, even though you do not realize a benefit from it since it is too late to amend that particular return. If you had 80,000 of carryovers in 2016 and did not have any capital gains in 2017, you generally will have to report 77,000 as your starting carryover amount in 2018 because 3,000 of them would have been used in 2017. The amount may be less if you did not have taxable income, were married filing separately in 2017, or had additional capital losses in 2017. The amount may be more if you had capital gains in 2017 to offset.
If you filed 2018-2021 with TurboTax, you do have access to your original filing, even if you did it online. You can get them by going to Your tax returns & documents and selecting Download .tax file for each year. For the current year, you can file the amended return online. However, for prior years, you will have to purchase the TurboTax software for that year to file the amended return. You can download prior year versions here.
You will need to attach an explanation statement to each amended return as to why you are filing. You should explain on the 2018 amended return that you missed the 2017 amended return period and how you determined the correct capital loss carryover for 2018 by taking the 2016 numbers and subtracting what you would have taken in 2017.
