analog - How can I add three AC signals?

I have three light sensors which outputs appropriate AC currents with respect to light intensities. I only have one pin left for my ADC IC that I have on my board so I can't sample three sensors using my ADC. What I want is that I want to average those three light sensor's current by sampling their sum feeding that number into one of my ADC and dividing it by 3 using micro controller and so on. I need help with adding three AC signals. I can't find out appropriate circuit for it.