Lets say you are in the Daggerfall Covenant. The megaserver will try to put you into a campaign where the DC is outnumbered to help even out the factions in that campaign. Unless you have a specific guild or friends you want to play with that are already in a campaign; then it will put you with them.
A big reason ZOS decided to do a three faction game is to help balance the PvP. Three factions make it very difficult for one to dominate because they are always up against two other factions. If one faction is dominating, the other two can always temporarily ally with each other to take out the dominating one.
And i dont remember where i read it but i believe ZOS stated you wont be able to just reroll and join the winning side. Plus campaigns only last around 1-2 months and then start over.
I agree that one really can't win for to long but its far fetch to believe tht two factions to team up. would it be smart, yes. but there r too many players who can "do everything themselves, and more importantly u make it sound as if all the groups in a faction will work to defend their area. each guild or group of friend may defend their keep and tht about it unless they ally with another guild type thing." I could see your point if all the ppl in a faction had the same goal or over all leader and they followed his/her orders. but most wouldn't even if u did had a solo leader
I could see people actually rolling the weaker alliance for the sake of an easier path to emperor as well so that could also work as a balancing mechanism. I am sure the developers have thought of many balacing mechaisms as they have done with the individual balancing of character levels after level 10
I always liked the way Aion intended to balance the 2 factions in their servers early in Aion's US lifecycle. The third faction were npc's and could be controlled by GM's. I never thought the npc faction was strong enough to really sway the game one way or another. Eventually, one side or the other would have an exodus causing major imbalance and long leveling times prevented new players from really helping their faction. Pretty much killed the game for me.
Bethesda is going to have their hands full trying to keep pvp and "winning" beneficial without stalling player progression for the "losing" sides. Like a previous poster said, it will more than likely be 2v1 against the strongest faction.