I am sorry but I did not understand how to use JS Builder 3.
I used JS Builder 2 with ExtJS 3.x it was quite simple and well documented here
http://www.sencha.com/products/jsbuilder in Overview.
I tried to find some informations about using JS Builder 3 but fails.
I also want to ask:"Is it possible to use JS Builder 2 for ExtJS 4?".
I think no.