What is the purpose of the three cylinders under the engines of the Junkers 52?
The three cylinders hanging below each engine are the oil coolers - this page has a much closer shot which I've reproduced below.

Oil enters and exits the cooler through the two vertical lines and swirls around the hollow air channels. The cold air in flight moving through the tubes cools the oil, which is then returned to the engine.

Courtesy of eBay, here's a photo of an old, rusted-out cooler where you can clearly see the air channel tubes inside. (Using an oil cooler in this condition would promptly empty the engine of oil...)