I've had this exact fault before. Check the fuel pressure regulator vacuum pipe for fuel which indicates a split diaphragm. The reason for No3 always being the faulty cylinder is it is in line with the lowest part of the inlet manifold so the excess fuel collects there. Original regulators are fetching big money so I replaced mine with an adjustable one with a gauge (ebay special) and all sorted.

Hope this helps as it's easy to check and I'm pretty confident it's your problem. Best of luck.