In other games that offer WvW a large problem is population. There generally tends to be a few servers with many more players than the ones they're put against. This can be a huge problem in WvW and fair weathers are always quick to leave when this happens, which only worsens the situation. In ESO people will be able to play on either of the three sides simply by logging onto their other characters. This seems like a huge problem to me as all of the fair weathers will quickly stack on one sides as soon as things go south.

What mechanics will prevent this from happening?

Balancing algorithms from campaign to campaign and when that fails, bonuses for the outnumbered side a la Planetside.

No problem. fairweathers as you call them are easy kills. ive never played wvw before as it lacks purpose for me. But rvr or ava in this case provides maning to the fight and that brings out the best in people.

So far 3 way PVP works, like Planetside 2 or TSW. I dont think they need any mechanics, but maybe there should be different kind of ways to win, IMHO that would improve 3 way PVP. More ways to win... more people are doing different kind of stuff and therefore are more spreaded. 3 different kind of ways to win that should do it if asked from me.

Gears is how they can screw it up... thats why I wouldnt want to add new gears every month, and not to mention when new set of gears is added, old ones should be given/sold to new players.

You seriously just didn't say that? While I am no fan of PvP myself, WvW or 3 sided siege warfare is by far the best form of PvP in an MMO.