I'm sure you have had chest xrays but have you had a chest MRI done?
It is not common to get pneumonia three years in a row. One might suspect that you have underlying asthma and that when you get a viral infection each year, it triggers pneumonia –like symptoms. I would suggest getting a pulmonary function test while you are sick to see if this is, indeed, asthma. If this is not the case, another possible reason is that the local immune system in your lungs is not protecting you from the infection. This is usually an antibody problem and can be checked with a blood test. I suggest speaking to your physician about these possibilities.