What would cause fouling and misfire on #3 plug. Found #3 was misfiring with my timing light. Change plug new one lasted a day then started misfiring also. Plug wire is good because I swapped with #1 and still did it. Any Ideas. Plug was not wet when i took it out. just tarnished black a little.

That cylinder has some sort of problem. A compression check is the next step. Check all eight so you'll have an idea of how it compares to the other cylinders. This could be the result of bad rings or bad valve guides or seals. Do the compression and let us know if that cylinder is weaker than the others. The answer will determine the next step to take.

My recently rebuilt 350 motor is doing the same but on #8. I took a compression test and its fine. Its probably the valve seals on mine. It gets a crusty oily cake to it.