If # 3 is fouling, you may need a new oil pump. The diaphram may be leaking . Old fuel will not burn off as completely as new fuel . The plugs will always have a dry soot on them, should not be a wet soot after a good run. Are the plugs indexed and gapped to .028-.030? If they are not indexed properly, they foul quicker and affect performance. A good decarboning will help you out also.