I was a non-resident for tax purposes and didn't file in 2017, 2018 or 2019. I'm now trying to claim tuition credits from those years on my 2020 return. How do I do it?

You cannot claim credits for the years where you were not a resident for tax purposes. 

If you were an international student, didn't work in Canada, or did not have a permanent resident (school dorm doesn't qualify), you will not be able to claim the tuition credits. 

Similarly, if you are a Canadian citizen but emigrated from the country and declared non-resident status but attended Canadian online schools, will not be able to claim those credits since you were exempt from paying taxes.

I hope this was helpful