Remembered this thread from a while ago... I just finished the third week of 3-boxing my shamans in 5v5 with an MS Warrior and Holy Paladin. We went 2-9 (lots of heated discussions on proper strategy), 7-3 and 6-4 (2 losses against full season 3 team) for those weeks and now sit at 1564. My gear is pretty bad as I've been hoarding points for next season, so I've done that with just the PvP blues and Kharazan cloth epics. Sporting about 700 damage, 8000 hp, 8000 AC, and 140 resilience... I finally relented and got the Season 3 helm after our last game this past week, and I'm drooling for Season 4 to start since I will have enough points banked to get the 4-piece bonus right away. The warrior is in full season 3 (got 2k in 2v2 with a druid) and the paladin is mostly season 3 and some karazhan/zul'amon epics (highest rating ever around 1700).

We use the setup spots that Ellay showed in his video a while back. The warrior helps immensely against teams with healers that like to just sit back and spam heals, as he can run around with Freedom and shut them down or apply MS to a target. He can also chase after runners (especially bridge jumpers in Blade's Edge) and prevent rezzers. We lose mostly to teams that are able to insta-gib my low AC/low HP shamans -- typically 4 dps teams, and heavy caster teams (especially mages who tear through totems with ice lance). Rogue+Warrior isn't too bad since I get a BoP that carries me long enough to down something.

The first week I had a lot of trouble with target selection. I think I was trying too hard to coordinate with the warrior, often asking him to pick the targets for me. After that didn't work, I just went into point-and-shoot mode, picking anything reasonable (dps first) that I could manage to click on in the first few seconds and letting the warrior assist me instead. This helped immensely. The warrior can intercept to my target for a stun and start spamming hamstring for an improved hamstring proc.

Based on the limited experience so far (limited time since I have a warlock and priest to also play, and my partners have 9 characters between them), I'd say this setup is definitely viable and the loss of damage from a 4th shaman is made up for in the added utility of the warrior. But you definitely have to be on the same page as the warrior. We play a lot of 2v2 together with other characters, and that helps our communication a lot.

My conservative hopes for season 4 are that we can reach 1700 for most of the season 4 gear. The highest I've gotten any character solo is around 1930 in 5v5 on my priest (we usually hover around 1850 though), so I'm not aiming too high just yet.