It is stated that the citron, pummelo and mandarin are the three "primary" fruits, and all other citrus varieties ever produced are some combination of these three primary fruits.
That's basically true, but not entirely completely true.
A few of the cultivated citrus have important ancestry from other original citrus species as well.
For example, lime comes from C. micrantha, kaffir lime comes from C. latipes (Khasi papeda), Yuzu is believed to have ancestry from C. ichangensis (or if not that, is closely related to it). Shikuwasa in Japan and Taiwan has ancestry from C. tachibana, a native original species to the islands that appears to be a closely related but distinct subspecies to mandarin.
Then you also have kumquat, which gave rise to calamondin.
Those are pretty much all the main exceptions, off the top of my head.
An example is the papeda family which was centered on Ichang papeda and has been proved completely wrong!
I believe all the papeda species are closely related. Or is that not true?