When should I notify ATC when I notice that I may have conflicting traffic?
I think you will find there isn't a correct answer for this, and that answers will primarily be opinion based.  That said, my opinion is that if you have ADS-B, be patient and let ATC do their job.  

Speak up eventually, of course.  Especially if you see a clear converging path on your scope.  The 3nm you suggested seems like as good a point as any to mention it.

It's a little different if you are VMC.  In that case I might wait even longer and attempt to acquire a visual first.  That way your reminder to them can consist of your assessment of the need to deviate, or declare that you can maintain visual separation.