EGT reading can be very confusing. Most have found that the position the probe is installed into the exhaust and its distance from the cylinder head will make a huge difference in EGT values. The exhaust cools very rapidly as it exits the engine. Most are installed about 2 inches from the flange and usually in a position that makes it easy to get to other stuff. More importantly to your issue is that they are all the same dimen from the flange.
See where that #3 is installed relative to the others ( distance from the flange) and if it is aligned any way differently. Or have damaged wire or even a bad connection back on the firewall where they are probably plugged into the harness. EGTs are a thermocouple and operate on a very minute voltage difference between two different metals in the probe and are very sensitive to resistance.
Also look up the 0-320 -XXX model number certifications and see which carb it should get . There is a mile long list of dash numbers which usually indicate jet orfice size ranging from 150 to 160 HP. Be sure you 've got the Correct carb dash number.
Another thought would be to swap the #3 EGT with another cylinder to see how it reacts.
Best of luck
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