i've 3 columns , all of those are editable h:inputs
I would like to calculate input3 onblur of input2 to be input2-input1
my main question is how do I know what will be the id of a spesific cell in the table
You may use standard behaviour and pass the parameters you need using f:param.
Simple example of dynamicall changing the tables you may found from the sources at http://labs.jboss.com/wiki/RichFacesArticleDataTable
and from the rich-faces sources (repeat component):