how to alter only one set of specific forms?
In the case you need, so you can easily use control flow in your hook_form_alter

You know that the hook runs everytime a form is willing to be rendered, so you can easily do something like

hook_form_alter(&amp;$form, &amp;$form_state, $form_id){

if($form_id == 'A_node_form'){...}//alter form A

if($form_id == 'B_node_form'){...}//alter form B

if($form_id == 'C_node_form'){...}//alter form C

}