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Is this going to be a problem? Can I let one cable hang loose?

It will work fine with 2 wires - the black and the gray - just lose the blue - cut off the connector and trim the cable to where it passes under the left hinge (once you have checked everything works fine of course).
 
It will work fine with 2 wires - the black and the gray - just lose the blue - cut off the connector and trim the cable to where it passes under the left hinge (once you have checked everything works fine of course).

Don't trim anything. All 3 cables should pass through. Just tape over the one you don't use. The Broadcom card has 2 connectors and the Atheros card has 3 connectors. In the event you swap cards you don't want to have to get a whole new antenna assembly.
 
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