We give 3 half shekels on Tanis Ester as a Zecher for the Machtis Hashekel. 3 because is says in Parshas Ki Tisa Terumah 3 times. Why would we give 3 half Shekels if when we actually performed the Mitzva no one was allowed to give more or less than 1 half Shekel? Seems like the Zecher does not match?
To a degree you are correct. In fact the Vilna Gaon ( Maaseh Rav 233) holds that there is only a need to give one half shekel. The general minhag however is to give three not only to remember the machtzis of the korbanos, but for all three times that they donated in the mishkan.