FastLED: Can't address led individually
The description in the link of where you bought the strip says:


  Description:
  
  
  LED Type: 5050SMD with External IC WS2811 （1 IC control 3leds）
  


So, this is probably as intended.

Note the application circuits in the datasheet for the controller IC (WS2811) [Page 5] where, for 5V, you can address a single LED, but for a 12V configuration, you must group the LEDs in sets of 3.