2 pipes going into the cold side of my water heater
I had a very similar system on my water heater and the additional pipe was for a heat recovery system from my ac compressor. Mine was installed in the early 80's but with ac compressors being so much more efficient now, the heat recovery system didn't work nearly as well as it did with my old unit. I have removed many additional pipes going into water heaters as they just aren't needed anymore and people didn't want them tied into their ac units for fear of failure. I also removed my additional pipes when I replaced my water heater. You could possibly find out that your's isn't even connected anymore.