When you fix a flat, you need to note where the tire aligns with the rim and tube. If you are removing the tube completely then lay it on top of the wheel and tire with them all lined up. Add a little air then find the hole in the tube. Then you'll know where to look, tire tread, sidewall or rim.

Also, you can see from one flat to the next if it's consistently in the same place. Don't rule out bad habits installing. I sometimes get a tube pinched as I try to get the bead over the rim. That sometimes takes a ride before it bursts and slowly lets air out.