Well you haven't said what 3 numbers you are expecting to receive and what 3 numbers you actually get?
Personally I would write a Processing program that just sends the same 3 numbers until I was happy that the communication to the Arduino works OK and the Arduino can do the right stuff with the numbers it receives.
Then I think I would write another Processing program that breaks the image as needed and prove to myself (without an Arduino connection) that it is doing what I want.
...R