2005 Dodge Caravan error code P0303

Does #3 cylinder have spark? If weak or non existant, a new coil may help. Or you might have a vacuum leak, when you pulled out the plug, did it have the same color as the others?

Get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs! In your case also spray around the end of the manifold near #3.