Hm. I'm sorry, but I'm going to have to disagree with you. You can't compare the evenness of the legs and how parallel they are in the first two pictures to the last picture. Again, because they are in different points in the stride, the could be (and probably ARE) moving very similarly. The third horse, may move the same as the other, but because he's in the down swing the stride, where his legs are moving down toward the ground, his legs are parallel, so they can strike the ground evenly. The other two pics however, the horses are in the UP swing of the stride where their legs are moving UP, away from the ground. Their legs are going to create different lines just because of how the horse is built. The legs can't lift as high in the back due to there being horse above them, but in the front, there is much more room for a lifted leg as long as the horse has a swinging, mobile shoulder. Plus, the legs will never be stick-straight all the time, no matter how good of a mover the horse is. The lines of the legs are going to differ.
Another thing I wanted to mention is that you are measuring two very different parts of the horse's legs. If you want consistent measurements of how much the legs are parallel to each other, then you shoulder measure the lower half of the limb. So make the lines even with the cannon bones on both the front and the hind legs. The forearm is going to be at a severely different angle from the hind legs at this point in the stride because the leg has to be bent in order to lift the leg, so comparing hind cannons to forearms is not going to be an accurate measure.
If you don't believe me, look at this video:
YouTube - Warmblood Stallion Extended Trot ridden by Steffan Peters
If you pause in that video periodically throughout the extended trots, you will see many different phases of the trots, including two that look very similar to all three of the pics that you posted, plus several others.
Hope you don't take this post the wrong way, I'm just throwing this out there for discussion!
I would actually argue that the 1st picture COULD be the nicest of the three if the picture had been taken a full second later. He's got the freest shoulder. He's got lovely power from behind. He's also got the longest stride of the three, so more to coordinate On the other hand, in the second picture, its taken probably a split second right before he was actually in full extension, so he looks as if there's little to no push from behind when there probably in fact is.
The third horse is actually closed in and has no place to really go with his extended trot. His legs might be better positioned, but in actually, the body of the first horse is closer to the ideal, because he's stretching upwards
You can't compare the photos with any degree of accuracy. They arent in the same positioning of footfalls. It's like asking a person to judge a flying lead change just in that first moment versus in a moment where all 4 legs might be off the ground.
Be interesting to see video clips of these horses and see how they went