Both handsets are identical. Pressure gauges are hose attached to the first stages of the regulators.
The diluent (1) and oxygen (2) pressure gauges are fastened to the inner side of the counterlungs. These gauges show the available supply of gases in the pressure tanks, where there can be up to 230 bars of diluent (or 300 bars if you roguishly ignore the recommendations of the manual) and 200 bars of oxygen. When diving, check whether either gas is being depleted too quickly, which could be a sign of malfunction. Of course, this normally will not happen and the pressure in the tanks will decrease very, very slowly during a dive.
That both handsets (3) are marked with the same number is not an error – both handsets are functionally fully equivalents and it does not matter at all if, for example, you change the required partial pressure of oxygen (setpoint) using the left or right handset; in both cases you will change the settings of the whole apparatus. On the handsets, you see all important information that you need during a dive, such as depth, dive time, required and actual partial pressure of oxygen and expected ascent and decompression time (TTS). You also adjust setpoints and other parameters on the handsets during a dive.