Have there ever been any conjoined triplets, and if so, did they survive?
WAG - you’d need the very small chance of an incomplete separation of cells which creates conjoined or identical twins to occur twice with the same foetus. Google tells me that conjoined twins occur in 1 in 50000 births. So triplets would be 1 in 2.5bn. And then you’re relying on complications not occuring, with complications surely far far more likely in conjoined triplets even than with twins.
I don’t think that calculation is correct, any more than the frequency of ordinary triplets should be the square of the frequency of ordinary twins. But I agree that we can expect the frequency to be small. The link that you gave gives the frequency of conjoined twins as 1 in 200 deliveries of identical twins. So we might expect conjoined triplets 1 in 40,000 deliveries of identical triplets. Triplets are rare, and my impression is that identical triplets are extremely rare.
Quite right. The odds on identical triplets are 1 in 600,000 births. So that gives odds on conjoined triplets of 1 in 24bn.