OS doesn't make a #3 glow plug. They make an A3 and a P3. Now Enya makes a #3 and this is an excellent plug. I use them in all my glow engines.

If an engine runs well with the glow driver attached but slows down or quits when the driver is removed it indicates one of two things. Either the plug is too cold or the nitro content is to low. An OS#8 glow plug is not too cold so his plug is fine unless it's worn out. He didn't say how long his fuel had been sitting around or what nitro content it had. Old fuel could be contaminated or some of the volitiles might have evaporated. This would make the engine perform like he describes. I would use at least 10% nitro but they will run well on 5%. Start with a new can of fuel and a new OS#8, A3 or Enya #3 plug to eliminate all the obvious potential problems and work from there.