The tire got flat the third day of changing it
How much air did you lose? What types of tires/tubes/rim strips? What tire pressure did you start your ride with?

A few possibilities:

1) Did you replace the rim strip/tape in each rim when you replaced the tubes and tires? 

2) You may have made a small hole in the tube when installing either by improperly using a tool to reseat the tire or by catching the tube between the rim and tire bead.

3) You may have a slow leak caused by something you rode over; a thorn, glass, etc. Or you could have a pinch-flat caused by riding over a curb or rock.

4) If the tubes have removable valve cores make sure they're not loose.