If you must be rebooked due to a missed, you should assume that you will be delayed for at least 24 hours and possibly several days if UA doesn't fly into your destination on a daily basis due to COVID-19 schedules. Unless you're going home or plan to stay with family (and relatively close to the airport if they're planning to meet you), I'd definitely take the three hour connection. With separate tickets, I'm not sure three hours is enough tome for comfort.