My cursory investigation didn't turn up any ISDG rules but I'm mostly interested in how we get to the finals. Is it the last three alive, highest final score or some kind of vote.

Well if it goes that far (i dont think it will) its the 3 with the highest score however what will probably happen will be the last 3 alive.

What if someone grabs culture/space race first? Is he automatically qualified together with the last two alive?

they're qualified with the next 2 highest points that goes through.

So one idea would be to play turtle or milk the game for score adn not get involved with any fights.

Any body a good milker

Well there isnt much point in milking the game, milking generally implies that you can win at any time you like but you want to keep making your score bigger. If we can win anytime we want id choose that over anything else.

Pulling a culture victory out of nowwhere would be awesome, though I don't think it's a safe bet.

no it isnt and we havnt got the civ for it either, its a bad idea because as soon as someone is about to win by culture everyone gets warned about 50 turns beforehand so you just get dogpiled.

That's true, I was just day-dreaming :p