My 3v3 Arena experience.

My highest rating was 1640ish, but it was basically pure luck on the comps I was facing. I enjoyed it for a day, and then the next time I logged on to do Arena's I dropped like 7 straight before winning again. Ended the day at 1505 I think, and I haven't jumped back into 3s since. I've put around 200 games in and I typically stay in the mid-high 1500s. I outgear that rating since I have an 1800+ (1890 max) rated 5v5 team, so I can overpower lots of the lower ranked teams and score easy wins. Then I'll just get owned by rogues/warlocks again for losses.

4 shamans + 1 healer is fun. You're not going to do 2 shamans + 1 healer very well in 3s, and playing 3 shamans is tough because you're doing everything. I doubt 5s would be very good 5 boxing, 4 is really a sweet spot because our 4 shaman burst can insta-gib someone on demand while allowing that last guy to help with the various tasks needed to win; like healing.