The first thing to do is to identify how they are fouling look at the plug is it oily, carboned or full of crust. If you are having oil fouling the fuel isn't the problem. If it is carboned up fuel is the problem. If it is crusty you might have a leaky head gasket and it could be sucking coolant. You should have a compression check done first. Check the compressions, if it is low squirt some oil in the cylinder and check it again. If it goes up after the oil, the rings are bad, if it stays the same it could be valves or head gasket. My experience is that when you have cylinders that are next to you displaying the same symptom it is usually a head gasket but you haven’t provided enough info to determine this.