FCC code, I believe will have overlapping spectral content because of the choice of weighting and code length. I found two papers that I think can help you out changing the fiber gratings.
Gives a great overview of SAC-OCDMA
You will need access for this paper but it explained the coding of FCC.
If you want to make a 3 user system I think it will simply amount to removing 1 grating on the transceiver side and you will have to choose the the remaining grating’s parameters to coincide with FCC.