This is an example of switching between 3 preamps from the GCX page:Multiple Preamps
You need to split the signal into the inputs of all 3 preamps. As long is it is buffered, you can do it with Y cables. If you use the GCX Guitar In, then you have Guitar Out and Feed Thru to get 2 and use a Y-cable for the 3rd. Then use 2 loops as A/B switches to select between the 3 preamp outputs as shown.
If you print the blank GCX worksheet, draw it up and post, someone can check what you're doing.
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