They're just like buses: you don't have a puncture for over a year and then 3 turn up at once. Saturday's was in the back tyre, yesterday's in the front and today's in the rear again. The two I already repaired were both due to small chips of flint, and I expect today's was too (unless it's just my previous repair failing), and that I cycled over a patch of chips at some point recently and they've been working their way through the tyres since. I picked half a dozen other small chips and pieces of glass from the front tyre yesterday.
All in all it's clear the tyres are getting a bit old and worn and aren't as resistant as they used to be. So I took the bike to the local bike shop and asked them to do me some new Marathon Plus tyres, supposedly some of the most puncture resistant ones available. New tubes (since the rear is already doubly patched) and some new front brake blocks while it's there and all in all it's going to be quite expensive, but there you go. Still, it's cheaper than running a car or getting the bus every day.
Of course speaking of buses I then had to get one to work. I waited about 20 minutes for the every-10-minutes bus, but apparently it was actually 35 minutes since one of other people waiting had just missed the previous one. And guess what, two turned up at once.