How can I get a B3 sound
And how can I enhance organs sound on modx getting different sound on stage then in my music room
There are plenty of B3 sounds in it... are you saying you can't find them? Or you're not satisfied with them?
Picking up from what Dave said, keyboards that don't have dedicated organ engines tend not to have the best organ sounds, because to do it properly, simply playing samples won't duplicate the way organs behave. (There are also phase issues, and a tendency to eat up polyphony at an alarming rate if any semblance of individual drawbar control is provided.) The MODX has two engines, sample playback and FM synthesis, and neither is ideal for emulating organ sounds. Plus there is the difficulty of emulating the rotary effect and overdrive... which is tricky enough to implement that the most highly regarded pedal that creates that effect is $500 just to do that one effect properly, and plenty of people buy it. This stuff just isn't built into your modx. That said, the MODX has decent organ sounds for a keyboard that doesn't actually include any organ-specific technology (i.e. if you compare to Korg Kross/Krome, Casio Privias, Roland Juno DS... in fact, even the Roland FA which *does* have an organ engine isn't so great). Organ is tricky!
If you find the B3 sounds in the MODX unsatisfactory, short of adding another board, you have options of a pedal as Dave suggested, a separate organ module (e.g. Legend, Gemini) as Gary suggested, or connecting to a laptop/tablet running a better organ emulation (VB3, B5, Blue3). A nice thing about the MODX is its Zone Master function that makes it easy to mix internal and external sounds.
Lastly, what do you mean about getting a different sound on stage than in your music room? Are you satisfied with the sound in one location but not the other? What playback systems are you using for each? The amp you use and whether you're going mono or stereo can make a difference.