I've gotten the flu three times in the last few months and its last 2-3 weeks at a time .is there an explanation for this ?

Influenza: It is very doubtful that you have had influenza 3 times in a few months. There are many different viral illnesses with similar symptoms and you may have had these, or you may have been infected with 3 different strains of inflluenza a, b or parainfluenza. If you were not tested to diagnose these you may not have had true influenza. Another possibility is that your immune system is damaged.