Unless you systematically mount the tire in the same place, ie. label at the valve, I suspect that all three flats were caused by the same shard of glass or similar embedded in the tread. You often can't feel these running your fingers along the inside of the tire because they withdraw back into the tire itself, then when riding get pushed through.

Search for the offending object from the outside by rolling the (removed) tire between your fingers to open any cuts, and see what's hiding there. Odds are that you'll mind multiple tiny shards which you can pick out with your fingernail, or a small pick of some kind.

It helps to have a reference, so you know where the valve is on the tire and can line up the tube along the tire and localize the area to check, but I always take a moment to check the whole tire while I have the chance.