Originally Posted by
Sykes
United's Arrivals Lounge outside of security on the baggage claim level of T3 has something ridiculous like 15 showers in it, but it sounds like your transiting SFO rather than arriving there so you probably wouldn't have access (although it might not hurt to ask).
Haven't used this in a while, but it's a very nice facility. If you have the time, no reason why you can't go out and then pass back thru security, even easier if you have pre-check.