yep he said it all, they are each games in their own, however each comes with an expansion campaign, areas (a whole extra world), weaponry items etc. and proffessions.
recommended to buy all 3 but if needs be buy them in order, they kinda lead together and there seems to be a lack of confusion this way. also, the first campaign you get 4 charcater slots where the others you get +2 so if you had them all you would have 8, the 1st and second 6 or the 2nd and third 4.
i have nightfall but hve yet to play until my comp is up and running, and i only began factions skimming the surface. im oldschool lol. but although i would be byast, i would recommend getting them in order if you cannot get them at once.
also depends what type of play you prefer, for instance, buying the factions game you will have 2 char slots and can only be a rt/as or as/rt (assasin and ritualist) which were the two chars released with that particular game. same goes for nightfall i think. where the first game has 6 proffessions for your 4 slots, so you can have a necromacer primary with 5 other different secondarys. as you buy the other games you can however mix the newer with the older. this also is a reason to buy the first game
hope i helped. Luke