It likely is one of two things--a three cylinder single stage compressor (most likely), or a three-stage compressor.
In the single stage version the cylinders are generally the same size and operate in parallel, producing reasonably large flow and quicker pressure buildup. For the equivalent CFM, the three cylinders, acting in sequence on the rotation allow a smoother and smaller torque requirement than would be the case with fewer cylinders.
In the three-stage version, the biggest cylinder’s output goes to the input of the (generally smaller) second stage which in turn feeds the third cylinder. This configuration is used for more efficient creation of high pressures, i.e. more than 125 psi, which you probably don’t need. Also would require higher rated storage vessels and connections.
Check that the tank has an ASME or equivalent pressure rating.