I woud think that one fruiting plum trre would pollinate another fruiting plum tree. I also have a Shiro plum tree and a Yellow Egg plum tree that cross pollinate ok.

They probably could pollenate each other, as in their pollen is compatable. Problem is bloom times listed. Shiro is March, Elephant Heart is April/May, Toka is May. So you might get a little bit of overlap between the Toka and Elephant Heart. But you are more likely to see a succession of blooming with the three trees, not so much them blooming at the same time.