Try replacing the distributor cap? Could perhaps be a hairline crack - or would that only manifest itself at high revs?
Also if a carb has been off, it's very easy to leave a wad of paper in the manifold (been there, done that... 😳).
Are the carbs connected? Easy to have only one throttle actually working.
If the manifold has been off are you getting a water leak into cylinder 3 from a faulty manifold gasket?
All I can suggest is doing a thorough go through of everything. Also, take the plugs out and turn the engine over in case 3 and 4 are flooded. Valves all appear to be wotrking OK if you've checked the tappets so you can probably rule that out.
Good luck - sounds infuriating...
Andy