Is there something "easy" as Slick2D just for 3D?
I did a few games with slick but now I want to try 3D but I dont know how to start???
"easy" is a bit exaggerated about 3D in Java because of the lack of some kinds of tools (WYSIWYG editors, full toolchains with system to compute spatial subdivisions for dynamic meshes, etc...) but Rebirth and ra4king's suggestions are nice. Do you already know LWJGL or JogAmp (JOGL, JOAL, JOCL)? It is a matter of tastes but personally I prefer feeling good with low level APIs before switching to higer level ones.
JMonkeyEngine has a nice community and very good tutorials for beginners but I still find Ardor3D more reliable.
And is there for 3D the same ressource problem as for 2D games??
What do you mean exactly? If you talk about models, I have to say that it is harder to find nice 3D artworks than in 2D.