3 Days -- this if for the test required to fly to the US. If your flight leaves on Friday to the US you can test anytime starting on the Tuesday (or Wed/Thurs); they just simply count the days.
72hours -- this is the requirement for flying back to Canada, and trust me, it's to the MINUTE that they look for (I have witnessed numerous individuals be denied boarding because they were outside their time; even 14mins outside the 72hrs). And it's based not on your landing time but your departure time of the actual flight to Canada. So if you are connecting, that last flight heading to Canada is what you have to time for (I factor in timezone differences when I'm connecting to make sure I'll fully covered). So if your flight to Canada leaves at 5pm on Friday, you can test anywhere from 5pm Tuesday forward (anytime Wed/Thurs since now for sure you are less than 72hrs).