Hope you get to read this. Looks like you replaced the # 4&5 plugs that were bad. Almost conclusive that number 3 the one in the middle has some kind of short to ground. Especially if previous owner changed the two same plugs reciently. Now time to borrow or buy a cheap digital meter $10-$20 should do it or borrow one. Remove wires from # 3 (plug in the middle) and you will probably find it goes to ground from the connection on the glow plug. It is not supposed to and is most likely why your #4&5 plugs burnt out. They will burn out again if that #3 plug is shorted and you do not change it. Another quick test instead of the meter is to take a 12 volt test light from the battery positive terminal to the connection on the #3 plug with the wires off. If the light comes on the plug is shorted. Post how this works out or if any other concerns or clarification required about this problem. Your engine is trying to start on only three glow plugs at best right now I believe and thats pretty hard when engine is cold. The numbers you posted where the firing order of the engine not the cylinder number . No harm as was able to establish what plugs you replaced anyways.