The 3 X's are the symbol of Amsterdam, you can see it all over our lovely city. They do not mean XXX, and they are actually, not even X's, they are St. Andrew's Crosses or Saltires, like the ones on the scottish flag. There are two explanation for them usually, the touristy one and the real one.
The touristy version states these X's represent the 3 dangers of Amsterdam in medieval times: Fire, flood and Plague. The real origin is that these were part of the original coat of arms of the Persijn family, lords of the Amstelledamme in the 11th century.