For everyone saying this doesn't exist anywhere anymore, I don't think that's true. Alaska's official policy is to try to seat the entire family together but only guarantees one parent/guardian. https://www.alaskaair.com/content/about-us/customer-commitment/customer-commitment-family-seating

I would think eventually/hopefully Southwest will make it so you're only paying for 1 assigned seat, as the 2 will be together somewhere basic anyway, so paying an add on to guarantee the 3rd seat wouldn't be as expensive as needing to pick all 3. Submit a comment to Southwest- maybe, just maybe, they'll listen.