You will need to amend your return for the year in which the loss occurred. Per the IRS:

Casualty losses are deductible in the year you sustain the loss, which is generally in the year the casualty occurred. You have not sustained a loss if you have a reasonable prospect of recovery through a claim for reimbursement. If you have a casualty loss from a federally declared disaster that occurred in an area warranting public or individual assistance (or both), you can choose to treat the casualty loss as having occurred in the year immediately preceding the tax year in which you sustained the disaster loss, and you can deduct the loss on your return or amended return for that preceding tax year. 

Casualty, Disaster, & Theft Losses

I filed my 2017 return on or about April 15, 2017. Has the 3 year deadline passed to amend this return or is there an extension until July 15, 2020, due to covid? Thanks for your help!

Are you referring to your 2016 or 2017 return?  

Your 2017 would not have been due until 2018.  

If you are referring to your 2017 return - due April 15, 2018 - the 3 year mark would be April 15, 2021.

I originally claimed a deduction on my 2017 return for the small amount of repairs that I was told about and had performed in 2017. Substantially more damage was discovered in 2019 by a professional contractor. So, would it be best to amend my 2017 return? Alternatively, could the damage discovered in 2019 serve as a deduction on my 2019 return? Thanks for your help!