It can be one, or a combination of three things that I can think of.

1) The spark plug tube seal on your valve cover for the #3 plug is worn, and oil is getting to it thru the plug tube. This is a pretty easy fix, just get a new spark plug tube seal, and swap out the worn out one on your valve cover.

2) Oil is getting past your valve seals on cyl#3, and into the combustion chamber, which could explain the oil on the plug. If this is the case, replacing a valve seal requires a special tool.

3) Oil is getting past your piston rings and into the combustion chamber. If this is the case, replacing rings requires removing the head and oil pan, and taking the piston out of the block. You may be able to remedy this problem by running a higher viscosity oil, which won't get past the rings as easilly as a lower viscosity oil.

that's all I can think of at the moment.