I'm w/ Dave on this one. I would borrow a carb from someone, and swap it out.
This will atleast rule out the carb.
Then go onto wires. This is if it is just those 3 cyl fouling every time. There isnt to much after the plugs and wires that would only affect 3 cyc.
The other thing is not to change to much at once w/o testing between each time.
I know it sounds like common sense, but that has burnt me when working on problem w/ my camaro.
Good luck w/ it.