A 10k swim at roughly 1.5 hour IM pace (I'll slow it down from elite level since I gather from your post that you are not a swimmer) is about 4 hours and 15 minutes, assuming breaks and aid stations. 160k bike (100 miler) at 36k/h is, what, 4.5 hours or so. A marathon after those two things would be hell, mainly because the swim is no longer a warm-up for the rest of the day, but an actual athletic challenge.

A 10k swim is not totally unheard of, in fact it's not uncommon when you get into the open water arena. 5 k OW is more common for rec/masters swimmer, but there are swims from 1k to 40k+, recognized by FINA and your local swim federation.

I too know more people who have done 100k/mile bike rides (less support/logistics required, easier), but I think the guys who have done a 10k swim (wetsuit illegal, by the way) are pretty tough.