How will I tell which is which?

I received these three 10 cm cuttings in the mail today, it was supposed to be 1 Macrogonus and 2 Bridgesii but they weren't labelled. Do I just say the two that look similar (7 ribs on the right) must be the Bridgesii and the one on the right (6 ribs) must be the Macrogonus, does that seem right? Does that one even look Macrogonus, it's the first one I got so I haven't got the slightest…