hmm. Looks like there might be more than one issue here. Can we start by having a look at your cable data, just to see if there is any issue with it. Here's a link to my CGN3 cable data as an example.
This is found on the DOCSIS WAN page of the CGN3. The power target is 0 dBmV on Downstream, and 37 to 40 on the upstream. The Signal to Noise Ratio should be in the region of 37 to 40/41 dB on the downstream.
Can you log into your CGN3 and copy and paste the downstream and upstream tables into this thread. Just the tables. Don't include the IP data, etc that sits above the Downstream data.
In a way I'm not surprised at different data rates with different devices, as every device has an operating range for the cable power and Signal to Noise ratios. In theory, you should be seeing the same rates, but hopefully your cable data will explain part of that.
Edit: I keep forgetting about this as I don't have this particular issue to deal with. The CGN3 has a firmware bug which causes it to throttle down the download rates when it is connected via wire, to any other device that has a 10 / 100 Mb/s port. If you look at the back of the CGN3 and look at the port LEDs you can determine what the device to device interconnect data rate is. If the port LED is yellow/orange its running at 1 Gb/s. You won't have any problems at this interconnect rate. If its green, its running at 10/100 Mb/s. The CGN3 will throttle down your wired download speeds. I believe that it also effects your upload rate as well. Solution: place a gigabit switch between the CGN3 and any wired devices that only have 10/100 Mb/s ports, until a firmware fix is issued.