So in Lich King, it's nearly impossible for three DPS to survive long enough to get a kill on a team with a healer, even with CC. I agree to this. The team with the healer will play conservatively and prevent damage where possible.
In cata they will do the same thing, but they will have vastly improved health and burst will theoretically be lower. It could be argued that the 3 dps team is even less likely to score a kill.
However they also have the increased health working for them. Even if a healer goes oom quickly though, these three people are going to have to survive through 2-3 health bars' worth of damage, without being healed themselves. I'm not sure they can do that. Granted I have no idea but I know even if you put my pally healer (
http://www.wowarmory.com/character-s...&cn=Gl%C3%B8ry ) with 2 DPS and put me up against a triple DPS team and only gave me half mana and told me I wasn't allowed to drink or use Plea...I still think I'd win 90% of the time. And that's with Wotlk burst. (ahh, but that argument works both ways, because that same wotlk burst and low hp pools work vs the triple dps team too...so really there's no way to know)
to sum up I think that it will likely be 2 dps + 1 healer vs 2dps +1 healer. whoever goes oom first loses, since you can't burst a guy down.