Admins regularly join sever 3! I always see
@WhoisDan join the server.
Admins have lots of requests to deal with; 'how do I unlock my car' 'RDM in athira' etc. They have to prioritise different things, don't forget they also want to play the game just like everyone else.
Admins play on all 3 servers and do admin patrols on all 3 servers. To have an admin available exclusively for each server all the time would involve recruiting more admins. This would be a long process of applications>refining>interviews>deciding>training. Don't forget not everyone would be suitable as an admin.
If you have an issue in-game with no admins on, then report the player. If it's more urgent, then I sometimes send a teamspeak message to an admin their patrolling channels near the help section of they're on another server and ask them to check it out; there's no rule against it but obviously it has to be fairly important.
Players aren't there to enforce rules, you can only guide people to follow them. The most any player could do to a rule breaker would be to imprison them as a cop/BH or zip tie them in a car if it comes to it. Even then you have to be 100% they're actually breaking rules, so be careful.
Additonally as Droge said, server 3 is a second overflow server. It would be priority to patrol server one and then two and three for the admins, would it not? In that event ensure you are recording.