Can I add this unit to an existing Mitsubishi Model#MXZ-3C24NA2? There are two indoor units

Yes, this air handler model is compatible with the MXZ-3C24NA2. However depending on the BTU rating of your other two air handlers, the unit may or may not be able to efficiently operate with a third air handler. A general rule of thumb is that the BTU ratings of the indoor units add up to be within 30% of the capacity of the outdoor unit. For assistance in this, please give us a call at 1-888-376-5487.