Recursive Workflow

Hi all, 

I want every 3 hour check in crm if some fields is empty or not.

Below is my workflow. (but something was wrong)

In first step I add "Timeout 3 hours"

In second I check if "marketcode" filed or "person" filed is empty then update "usebudget" field.

In third step I send email (just test)

And in last step I again start my workflow as child workflow to repeat every 3 hours.

I don't know what to put  in "START WHEN: " , because my workflow like a job just every 3 minutes checking some conditions and updating.

Thnaks.