They were triplets to start off with. But, It was 2 girls that conjoined. The third (boy) was perfectly fine.
So most likely there were two eggs fertilized by two separate sperm, and one egg’s early (very early) embryo irregularly split into the conjoined twin girls, with the separate egg developing a single embryo that was the boy.
With identical twins or triplets (or more) one egg develops a an embryo that very early (usually within 9 days of the formation of the egg into an embryo) splits into identical siblings.
With fraternal twins or triplets (or more) more than one egg is fertilized by separate sperm with each developing completely separate embryos.
So that would be two identical triplets and one fraternal.