A three cylinder compressor can be a single or two stage. The simplest way to tell is to count the air filters. If each cylinder has its own air intake filter, it's a single stage. If two of them do and the third has a pipe connecting to the other two, it's a two stage. In the case of small air compressors, there's no such thing as a three stage. A single stage uses one or more pistons/cylinders to pump air from the atmosphere directly into the tank. A two stage uses at least two cylinders, a larger one takes air from the atmosphere and fills a second cylinder that is smaller in displacement, raising the pressure in the process, that second cylinder then compresses the air further, raising it to tank pressure. A single stage is cheaper, but less efficient than a two stage, a two stage also tends to run cooler and last longer.
Putting things into perspective, the human heart is a two-stage design with two individual complete "pumps."