The problem with T3 is it's a lot easier to micro 1 Uber unit and some escorts (and get them where you want them) than it is to mass and controll an equivilent number of T2 units.
Having a bunch of T2 units can also be an advantage in that regard, since you can split up your T2 units whereas with one super unit you can't. Want to attack your opponent's base from two sides at once? How about split apart when you've broken through his lines so as to do as much damage as possible? You can't do that with just one T3 unit.
The fact that T3 units are easier to micro is a benefit for them, but they're also expensive and require a lot of time to get to. If there were no benefits to going T3, then nobody would! :)
the ocrone and the krog are fine..
but personally i really dislike the other L3 units... they are cheap!
One Razorback (my personal favorite T3 unit, and in my opinion the most cost effective) costs 3626 Metal.
It can be defeated by:
3 Bulldogs (944 metal/pc, 2,832 metal total) = 794 metal ahead for the Bulldogs, with one Bulldog left standing.
3 Penetrators (1,129 metal/pc, 3,387 metal total) = 239 metal ahead for the Penetrators, with
all three left standing. (This was tested with the Penetrators getting the first shot in, which they should with their much longer range.)
10 Zeus (354 metal/pc, 3,540 metal total) = 86 metal ahead for the Zeus, with five Zeus left standing.
9 Recluses (375 metal/pc, 3,375 metal total) = 251 metal ahead for the Recluses, with three left standing.
I tested all of this by loading up a map, putting a Razorback in the corner, giving it computer control and then by having my units run after it. After the battle, I self-d'd whatever remained and sent Farks in to clean up the mess. Massed T3 units will obviously make it harder for T2 units to get close, but at the same time you could also mass T2 units or back them up with artillery/snipers/aircraft.