Frankly I don’t know which to learn, 2 or 3? 2 may be more reliable, then again if I learn 2 then what happens once 3 becomes the stable version?
If you have to make something right away and need backward compatibility(for low-end machines), jme2 may be an answer.
If you need advanced resource handling, performance gain and new functionalities, choose jme3.
jme2 and jme3 are quite different from each other
https://wiki.jmonkeyengine.org/legacy/doku.php/choose-jme2-or-jme3