白马王子 Slow Chinese Stories | Chinese Listening Practice HSK 3 

当电灯泡 I have seen this expression before. I think it means "be a light bulb" only when actually talking about light bulbs. When talking about people, we could say "be a fifth wheel" (even though there are only three people, and not five). More commonly, we would say: "two is company, three is a crowd". So 你想让我当电灯泡 could be: "that would make me a fifth wheel" or "that would make it 'two is company, three is a crowd' "