I think that you want to have the user select field on the company page but it may be easier to do if the the company and agent are selected on the user edit page.
So on the CPT for user there would be 3 relationship fields, one for the User, one for Company and one of Agent.
You can get the current user and then do a query on the user cpt to find the page associated with that user.
Hope that makes sense and helps.
