Let me get this straight, I've lived in the US for 3.5 years on F1 Visa. Recently I received my canadian PR. Does that make me eligible for NEXUS?This is not accurate.
The requirement is not 3 years as permanent resident, but rather includes any other forms as temporary resident such as TFW or Work Permit etc -
In order to apply for Nexus you have to have PR but the time spent in Canada as TFW / Work Permit counts -
Including myself many many others have received Nexus Basis recent PR award but majority of time spent in Canada basis Work Permit.