3rd level stances:

Diamond Mind:

Pearl of Black Doubt: Less useful than you may think. Bonuses only last a single round, and if they're going to be missing you, more AC isn't going to do much more good. And if they are hitting you, you're not going to be piling up bonuses.

Iron Heart:

Absolute Steel Stance: Extra speed and a +2 AC bonus if you move around. Not bad, but there's better 1st level stances to be in.

Stone Dragon:

Roots of the Mountain: Well, if you could keep it up while moving, it would be pretty decent for a battlefield-control specialist. Any creature that goes into your threatened space gets a -10 on Tumble checks. That makes it a lot harder to tumble past you to avoid AoO's. DR2/- isn't bad either. However, you have to plant and stay put for it to continue being effective. This makes it far less useful.

Tiger Claw:

Leaping Dragon: There's a ring which does pretty much the same thing this stance does. Belkar has it, in point of fact. Remember the cardinal rule of choosing precious stances, my young padawan: Never spend a stance on that which you may purchase for gold.

White Raven:

Tactics of the Wolf: Extra damage while flanking equal to 1/2 IL. If you can reliably set up flanking opportunities, this has a lot of potential. Remember, your buddies get this too, so flanking rogues are now doing even *more* damage. This is like half-again what Craven can do for them. If you've got several melee users who like to flank, or if someone has Clarion Call or Island of Blades or some other way of making flanking happen easier, it can be pretty nasty. Requires some forethought and setup, though.