1. Ensure all software updates are down to each unit. Procedure already talked about. Get a larger SD card 2 gigs you'll need it to back up your wpts, rts, trks.
2. You'll be running whay is know as an expanded network. The 4212 & 4208 mfd's each have three network connection ports and the 3205 one .... more than enough connectivity for your network.
3. Not sure if your running a NMEA 2000 backbone but if you are it needs to be connected to it's own power source. If you are and have the NMEA 2000 GPS 17x antenna then it will be plugged into this 2000 backbone. If you have the standard GPS 17 antenna the it will be connected to a power source and connected to your 4212 via NMEA 0183 connectivity.
4. If I were your I woukd connect your radar and XM weather modules to the 4212 ands your sonar (GDS 22) to the 4208.
5. Next ensure your network is functional .....
a. connect the 4212 to the 4208 and the 4208 to the 3205 using the network cables.
b. power the 4212 up first then the 4208 followed by the 3205.
c. your simrad AP has to be connected to one of MFD's via NMEA 0183 connectivity. My recommendation would to connect it to the 4212.
After you do that read pages 37 & 38 of your owners manual that came with your 4212 or 4208.
Because you have your AP & DSC VHF radio's connected to one your mfd's via NMEA 0183 connectivity if so you'll have to turn the DSC functionality on and ensure your comm ports are set correctly.
I have a question .... did the folks that built this boat install the electronics if so they did a poor job!