If you are talking about the Clark electrode sensor to O2... It has a Platinum-tip as working electrode; because its micrometric size, it can use a silver wire as both reference and counter electrodes. The Clark electrode has a PTFE membrane permeable to oxygen. The oxygen gas diffuses through the membrane and is reduced on the working electrode surface (cathode), generating an electric signal proportional to the O2 concentration.

Some clark type oxygen sensors contain an additional electrode (sometimes called guard electrode). The operation of this electrode is the same as for the sensing electrode, but it's only purpose is to keep the oxygen concentration in the electrolyte to a minimum. Typically before taking measurements you should remove the oxygen using this electrode until a satisfactory baseline is obtained (can take a couple of hours). From your question I believe this is the case, as you are talking about 3 electrodes. But as Francisco pointed out it is essentially a 2 electrode cell.

Kind regards, Mike