I know that isn't possible to use a custom H3 3D on another drone than P2 but Is it possible to use an H3 3D gimbal NOT CUSTOM on a P2 ?

Thx,

Not sure what you mean by "Custom"  As I understand the gimbal is provided in tow models.
One has the GCU and the gimbal and is intended for Non phantom copters with a DJI controller.
The other is gimbal without the GCU sine this is built into the Phantom 2.
You can use the H3-3D in case one with the Phantom you just do not use the GCU that came with it.
There may be some difference in the mount bracket but if need be you can get the P2 mounting bracket.