440 6 pack issues

The problem with triple carbs is.....that you can get 3 times the problems that you can get with a single carb.

The engine is getting idle air from someplace other than the centre carb, hence the t/blades nearly closed. Make the t/blades in the end carbs are closing fully & not adding air. Make sure the PCV is working, not gummed up. Should rattle when you shake it.