onyxboox.com, which is what it seems you are referring to, is
not "the Onyx homepage", it is the page of the US offshoot of the Russian retailer (who is the only source that sells Kepler variants under the name "Monte Cristo"). The actual Onyx homepage is
www.onyx-international.com
The main difference between the three appears to be that the MC2 & MC3 have Bluetooth (4.0) and "SNOW field" (their alternate name for a minor screen technology tweak that is available from other sources under a different name -- I cannot however remember what that name is, or the details of the tweak). They also do a waterproof (but BT-less &Sf-less) Kepler variant called the "Robinson Crusoe".
ADDENDUM: There also appears to be an RC2 that has both those features. I do not know however if it is available anywhere yet.