How to edit CCK form? How I change node edit form, which created by CCK module? 

Now if the form is submitted, CCK update some field in mysql database. I need to change this, because in one form input contains three values, and this values need to be recorded in three fields in databse. How can I do this?

Jimmy Ko said, that I can retrieve the submitted value in $form_state['#values'], but I don’t know in what .php file I can do this?
For achieving this goal you have to implement hook_nodeapi

In presave operation you can get desired value.

Example code you can get from the D6 release of the Examples for Developers module.