well, I believe there are loads of tread left, it's still as stiff as it was when I got it. I commute on it nearly every day, but distance is pretty short so I doubt it's at its end of life.

All three were nails, two were like an inch long. Today it was about a 3/4" nail smack in the middle of it. It's always my rear tire, too, which is why I'm thinking it needs to be replaced.

Or, like you said, it could simply be just bad luck. Don't know, but not having had any flats for over a year and all of a sudden getting 3 within a span of 5 weeks makes me wonder.