From least skill necessary to most -

Erebus
Oak
Sedna
UB
Rook
TB
Regulus
QoT (with 1.2 bug fixes she's probably going to the top of this list, but let's see)

1.  I'm not saying that you are cheap or bad because you play Erebus - there are some awesome vamps out there, and their skill level can be the same or better as that of an awesome QoT

2.  I'm not saying that an excellent Rook/Reg/QoT team can't beat an excellent UB/Oak/LE team - they certainly can

3.  I'm definitely not telling the newbies, casuals, or competitives to avoid Regulus or QoT

4.  Finally, I'm not saying Regulus is harder to use than, say Rook -the bottom four are loosely interchangeable

What I am saying is that the further down that list you go the more skill you will need to be successful in a high-level 3v3 on Cata. 

Just as important - the further down that list you go the better your team must be to understand and take advantage of your abilities.

When you come into a skilled 3v3 and ready up as TB you really need to know what an effective TB looks like, how that fits into your team, and how your opponents will counter, or you don't belong there as TB.

Random example:  You can be the best player in a game and pick Reg and up your win conditions 30% because your opponents are able to deflect the gank/suppression you selected for and you don't have a team that can push lanes and control the map (neither of which Reg can do very well against, say, an Oak).  If you can't make the kills/suppression and your team can't hold flags then GG

The bottom four DGs on that list are unforgiving - they will kill you again and again if you make mistakes with them, and they will keep you from winning again and again if your allies don't know how to play with them. 

They are powerful specialists in the right hands and on the right teams, but the TL;DR can be summed up right here:

Unless you are confident in the abilities of yourself and your allies you should not play Rook, Reg, TB, or QoT in 3v3 PUGs against competent teams which include Sedna, UB, LE, and Oak

(This isn't about current community level of play, it's about levels of mitigation and exposure inherent to each DG - I'd be surprised if it changed without dev intervention - Reg, Rook, and TB need some individual and team mitigation beyond what they have and QoT needs to be fixed)

Thanks for Reading

Kestrel