How do I exclude a booking class from a specific multi-leg-itinerary?

I know I want to fly from ORD on UA with two stops to ATL, and I want to exclude booking class N.

If I search with "UA UA UA", I get the required routings, but many includes a leg in N. If I search with "/ f ~bc=N" I only get direct and one-stop routings.

How do I combine the two restrictions? I've tried many variations, but all result in "bad specification" error messages. TNX