How to make a counter in JMeter

[Chen, Song]

Hi, 
  I have a scenerio, in which I need request 1 page, it should be
requested after every 3 rounds, so i plan to add a counter, and a "if
controller"(mod 3 = 0) to this request.

  How to make a counter, added 1 by each round?

Thanks

Chen Song