Out of curiosity, did you reach out to Garmin over phone or the G3xpert email inbox? A question to the firstname.lastname@example.org
email will generally get a quick response.
The one item that jumps out to me is that all three of these sensors (and probably only these sensors) use the same voltage source out of the GEA. Did you verify the voltage output from the GEA? How about the voltage back into the GEA from the sensor? These pressure sensors (provided they are the standard Kavlico sensors) output a simple voltage signal that is read by the GEA.
If you really want to get to the bottom of your issue, you would probably need to provide quite a bit more details on what you have tried and the configuration of your setup. At a minimum, you should provide the following information:
What GEA/GSU do you have?
What are the part numbers for the sensors in question?
What configuration settings have you configured on the display for the sensors?
Which pins on the GEA did you wire the sensors to and on which connector? Do other engine indications work?
Is your GEA communicating with the system?
What software version do you have on your GDU and GEA?
What troubleshooting steps have you taken already?
I understand the need to vent, but, honestly, working directly with Garmin is probably the fastest way to resolve your issue. If you have not provided all the information in the above questions, it would be hard for them to help you get to the bottom of the issue.