I'm not sure I'm fully comprehending your question/problem.
The FX assignment knob assigns the FX to a specific channel (or Master, fader, etc.). Therefore, assigning the FX knob to Channel 3 will only apply FX to Channel 3.
When you say you "get no effects none" when you have the FX applied to the Master output, have you tried other FX? Also, are you listening for the FX through an audio system connected to the Mixer's Master Out or through your headphones? If the later, be sure you have the Master cued in your headphones, not the channel. Otherwise, you won't hear any FX coming out of the Master.