I was on a flight once that had a similar situation. It was on Delta, not Southwest. I was delayed for close to 4 hours because our flight didn’t have a crew so they were flying in a crew from MSP. The original crew was involved in a weather related delay on their previous flight and they would have gone over their hours if they did the flight I was on so they needed to call for another crew. Even though my actual flight wasn’t experiencing weather issues, the delay was still considered a weather delay. Delta did give us miles (can’t remember how many), but they did that as a courtesy to keep passengers happy, not because there was some 3 hour rule.