In addition, certain permanent residents are also exempted from the three-year residency requirement.
However I couldn't find the exact list of conditions for being exempt. Does any website explain this in better detail?
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There's a link given on CBP website and link on CBSA website that elaborates. "Canadian and American citizens living outside of Canada or the United States or who have recently returned to either country and have not previously been able to meet the three-year residency requirement are now eligible to apply."
In addition, if you are a permanent resident of either country and have lived in either country for more than 3 years, you might be eligible (they will need to check your background, travel history etc. to confirm this).