3 devices may be too many to have on 1 Nmea talker. Does your raymarine autopilot have a Nmea output? I would wire that to the radar.
when wiring the furunos, you dont need to wire up the shield or the ground to chassis. you only need to connect to the yellow and green wires.
i would split the nmea from the lowrance to your autopilot and sounder, then have the nmea output of the autopilot send to the radar. from what i have read in the autopilot manual, you may need an additional cable for your autopilot if you want to do it this way.
otherwise you may want to invest in a Nmea 183 expander. like this one here. http://www.navstore.com/detail.aspx?ID=1260