Not all 3 have to respond all the time. It's a rotating schedule for having the overnight phone and the weekend phone (during weekend nights the SLA is relaxed to 1 hour).
And keep in mind, 20 minute response doesn't mean you fix the problem in 20 minutes, it means you respond in 20 minutes to the callout.
I think you're victim of an easy-going startup culture.
I’ve been oncall for escalations for like 15 years. That’s miserable enough, IMO frontline guys need fixed schedules and rotation if the volume is high.
I definitely cast my perspective on this and apologize if I came on too strong,
Sorry to say, you're right and he has Stockholm syndrome. It's a very aggressive schedule, must be missing holidays half of the time to keep the phone.