Three flats in five weeks IS rather surprising. I can see how it would seem discouraging! If it truly is a problem with faulty tubes. then the bike shop should have quite a few customers coming in with the same thing happening and I would think they'd dispose of that particular batch of bad tubes. At least return them to their distributor! I think I would buy a tube from another bike shop and see if it holds up. If not, then I'd suspect a problem with the tire and not the tube. How old is the tire? Where is the flat occuring? On the inside or outside of the tire? On the sidewall? It is always good to find the location of the leak and compare that to the tire.
Hoping you solve the problem and enjoy the ride.
annie
Time is a companion that goes with us on a journey. It reminds us to cherish each moment, because it will never come again. What we leave behind is not as important as how we have lived." Captain Jean Luc Picard