His doctor told him that his case was effectively unique... The chance of developing three different types of cancer is, well, pretty unheard of. ....
and then his wife went through a similar routine of three different cancers, and was similarly told that her case was unique
As you might have guessed already by virtue of having two "unique" cases, it isn't quite *that* rare. I mean, family factor aside, cancer is not that rare, and as (thankfully) treatments get better and more people survive, it also means that those people will get the chance to roll snake eyes again. And some tumors (like prostate cancer for instance) are really common.
Also, cancer therapy in itself can increase the risk of second cancers down the line. Chemotherapy and radiotherapy, obviously... but also procedures such as CT scans (and if you get cancer you're getting quite a few CT scans). Compromised immune systems also lead to increased cancer risk, and both the disease and therapies tend to lead to a compromised immune system
Edit: and a compromised immune system may in turn lead to different types of cancer which would normally be wiped out, to thrive.
And... well, I dont know your relative's lifestyles, but risk factors tend to increase the risk to more than one type of cancer, too.
As far as personal experience: first day in clinical practice in med school I met a guy with two simultaneous, distinct types of cancer. I too was astonished by the coincidence... and by the attitude of the internal medicine physicians and the pathologists, who weren't all that surprised. I think that both tumors happening at the same time did prompt a proposal to screen the family for common cancer causing mutations, but I dont know if they actually followed through.
I've met (and on occasion treated) more, since. It's not exactly usual but it's not unique. In certain circumstances it's actually an immediate risk that you have to adjust for.