I'm one of those that have found 3v3 more enjoyable than 2v2 and 5v5 for a couple of reasons. 3v3 was a lot less rock/paper/scissorish than 2v2 (and one can argue, even 5v5), and at the same time it's a lot less chaotic than 5v5 while maintaining some of the chaos as compared to 2v2.
In season one the typical cookie-cutter setup in 3v3 was a warrior with two healers, mostly shaman/paladin, while there's not much that could beat the warrior, paladin, shaman, priest, x, setup in 5v5 assuming equal gear and skill. As the season progressed the teams geared up and by the start of season two a good team would have pretty much all the season one rewards worth having as well as 5000 points worth of gear from season two.
I guess this post is aimed at the gear progression in arena that seems to force teams to exclude classes/setups to be able to compete on a high level.
As an example, my season one 3v3 team was an ap/pom mage (me), a warlock with a spec that changed more often than the weather, and a rogue. We maintained a high rating over a long period of time and ended up just above 2300 rating with healthstones as our only escape when we got low. The tactic was fairly easy and it was based on pouring as much dps on a target so that the healers couldn't keep up. Against a setup with one healer very few healers could keep up with the focused fire, and we also had few problems with nuking down the warriors in the warrior/two healers-setups. Usually I would die in the process as a mage with no iceblock is made of cardboard and teams that faced us a few times knew that I had next to no surviveability. However, I would not go without inflicting serious damage so we had the luxury of killing the target that would give the best setup for a remaining 2v2 with our rogue and warlock. In fact, the only setups that gave us serious trouble was the ones with both a warlock and a shadowpriest that could get me down in seconds.
Then season two came around and we tried with the same setup with very limited success. Against setups that was a breeze earlier we would struggle, and against those that we'd struggle but usually win against in season one, we'd lose (Thanks for the extra armor on our cloth ;-)). Typically where we'd take one down comfortably, we'd almost get him down, but almost won't win us much rating. There are a few things that comes on top of the gear upgrades that has hurt our team (fix to rogue poisons, pvp trinkets), but I feel that the added resilience, stamina and damage on new gear hurt us the most.
We kind of realized that our setup was ready to be scrapped when we faced a team with a warrior that did as much damage as two of us, and on top of that had a healer on him. The uniqueness of our team was something that we treasured and that we were quite proud of, but struggling for a 2000 rating isn't something we want after competing for the #1 spot in the battlegroup (especially not with the change to the arena points, but that's another discussion). Luckily for us, I've got a level 70 resto shaman with decent gear and the rogue/shaman/warlock combination seems to do very well, at least considerably easier than with a glasscannon in the team.
As a slight derail, is there anyone out there that has a successful tri-dps spec team?
Sorry for the length of this, I wasn't intending to make it so long, but to the real discussion. I believe that the top of the arena rankings will with time (more stamina and resilience etc.) exclude variations of setups that's out of the norm, more so than it has now. To me it feels that the reason that we have other top-ranked teams than cookie-cutter setups is because people in fact play all classes.
So, should anything be done about it, and if so, what can be done about it? I've seen blue posts saying they are looking into warriors, rogues and druids in the arenas. An incoming nerf to warriors seems to be the general idea of what's going to happen, but how can they nerf something without making other aspects of arena play (not to mention out of arena gameplay) stronger? For example, a nerf to MS could just make the healing teams even stronger.
I think high end PVP, much like PVE, should be exclusionary, to a certain extent. In 5v5, for example, there should be a requirement for healing, a requirement for reducing the effectiveness of healing (MS, wound poison, CoT), as well as some form of CC. Notice that none of these "archetype" requirements necessitate a certain class.
I also believe that this "exclusionary principle" should be more lax in smaller brackets, so that in 2v2, for example, it may be possible to compete with 2 dps, or 1 dps and 1 healer, but where the exclusion would come in would be the inability of 2 healer teams to compete. The 2 DPS teams also cannot be just any DPS, but typically must include either strong CC, to make it an effective 2v1, or strong, controllable burst, also to make the match quickly into a 2v1, where the lack of a healer on your side is no longer a glaring disadvantage, since either the 1 is a DPS, and thus without a healer just like you, or is a healer and lacks the damage capability to pressure you into needing healing.
In 3v3, I think the envelope opens up a bit, and the size of the team, combined with the "slowing down" of high-end arena PVP due to the increasing stats of survivability on PVP gear, require that a healer be present, since immediately taking out one member of an opposing team no longer presents the overwhelming advantage of doing so in a 2v2, especially since during the same time, most likely the opposing team can muster enough damage to critically injure, if not outright kill, one of your teammates.
I think it's healthy for the game to require these groups, and I think anyone suggesting that any combination of players (which I do not think you are, by the way) should be able to succeed at high-end arena PVP would be better suited to a solo-based FPS or RTS, some game where the initial starting conditions are more or less equal, unlike the inherent inequality presented by the differing classes in an MMORPG.
I see the 3v3 bracket as an awkward position for the reasons Karakas stated. It's large enough that healing and healing-negation really become force multipliers, but it's not large enough for a real "balanced" team design to emerge like the 2.5 Backline of the current Flavor of the Month. The only parallel I see to the current 3v3 game is that of 8v8 BF1942. Both do not allow for the full usage of every map/character aspect, but became the dominant competitive bracket simply because it was easier to handle teams of 8 and 3 then teams of 12 and 5.
Right now at the 5-man level, the fact that 4 DPS (usually with a 1.5 Backline) is viable shows the robustness of the format. I haven't really seen the 3-man format show that, although the fact that you ran a full DPS team well shows a hint of that.
As for changes to certain classes, I think the emphasis is too much on DPS (the Warrior in your example) and not on utility or viability under pressure. Warriors are the only class that gets better with pressure and have the "best debuff in the Arena" while Rogues can get close to matching MS with Wounding Poison, but don't do as well under pressure. Paladins versus the other healers is a similar case. The power of certain classes in the upper brackets forces you to either mirror-match or pigeon holes you into a counter that strategy that would cascade through your entire make-up. (Holy Priests as the second full healer instead of Druid/Shaman for example.)