I don't believe that would be allowed. You're allowed to book an open jaw itinerary with a domestic award. Open jaw for this purpose means that the leg of the triangle that isn't actually traveled must be shorter than the other two legs. In your example, either SFO-ORD or LAX-ORD would form Leg 1, and ORD-IAH would form Leg 2. But Leg 3 is longer than Leg 2; Leg 2 is 926 miles according to Webflyer MileMarker, whereas Leg 3 is either 1630 (IAH-SFO) or 1370 (IAH-LAX) miles. Thus this would be deemed a stopover, which I think isn't allowed for a United domestic award.