Is DDD even a cup size used?

Not really. It's a hold over from when the average breast size was a B cup & a D cup was considered large, hence double D was double large & DDD was triple large.

But now the average cup size is bursting out of a C cup & heading towards a D cup, so anywhere that isn't American continues up through the alphabet. So a DD is essentially the equivalent of a E cup & DDD an F cup.

the things you pick up in a long life: I also know how to measure an inseam, darn a pair of socks & if you give me a janome overlocker with a gathering foot I can make ruffles. What can I say, I paid attention in high school home economics