In a 3-city-game you're not allowed to build more than 3 cities. This is enforced by a ruleset that disables settlers, but allowes a new unit "emigrant" to be build. The emigrant is nearly identical to the settler, but it can't found new cities. You can still use it as (expensive) worker or to move population from one city to another.
Without that rule, you cannot AI a player when he disconnects, because the AI will start to build new cities. Some players think, you should be allowed to build a new city, when you lose one, but that rule is inacurate, and might leed to unfair tactics when two players work together. Plus, if you lose a city it is too late to found a new one anyway, so i dont see practical use for a rule like that, and it prevents the usage of AI. Ruleset to enforce 3-City-Game for AI.
In a 3-city-game it is very very important, where you build your cities. If you place your cities on bad spots, you've little to no chance at all. You should build at least one city with access to oceans to build ships later.
If you attack and conquer another player, you have twice as many cities. So it is a good idea to conquer somebody if you can, and its a good idea to defend well.
Since you cant expand, you have to grow your cities as fast as possible, so you should use rapture to get decent sized cities.
There is (should be) a port on pubservers only for 3-City-Games, with other defaults:
/startunits cccwwwxxx (3 workers to grow your cities faster, and 3 explorers to help you find a good spot fast)
/set disp 4 (makes it easier to find good spots)
/set specials 350 (To make it easier to find good spots for your cities. With 250 luck is too important)
/set revolen 2 (Length of revolution is more important than in normal games, since you cant expand while revolting, so it should be the same for all players)
/set diplcost 100 (6 cities against 3 is rather unfair, so preventing tech-exchange might be a good idea, since you cant counterbalance it with more cities)