modify text file

need a script that will run on a mac that will read a text file (either comma or tab delimited) and change the value in one field by merging it with values from 2 other fields.

so basically ... read each row... capture the value of 3 columns... build a new string of text... and rewrite the value of one column with that new string...

and do this for each row in the text file

script should be easy to modify

user needs to be able to easily define the text file for the script to run on.. so a simple gui...

also ideally would like to be able to change the script ... so not something compiled