Depends. There's a lot of stuff that could go on. As for sprinting, I'm not sure. I know that kb swings have helped my endurance level. (at this point, I'm going for pure strength, so no sprints or other stuff till I add more muscle).
Here's a couple thoughts though...
Do you have a deload week? You might want to take a look at your training. If you've been working 6-8 months at this, it's time to drop the weight a bit, and give yourself a bit of a break, or just a week off, and then start at a lower weight.
You're saying the third set. Of how many sets? How many reps? I get really good mileage out of 3x5 (starting strength), but please remember, squats are uncomfortable, not necessarily fun, and you have to keep your head in the game. A lot of this is mental! (of course, there's also 5x3x1, 5x5, etc).
Is this trainer someone who has powerlifted or knows how to squat? You might look at getting professional instruction by a powerlifting coach...or watch the elitefts "how to squat" series on youtube if you want to check your form to be sure you're doing it good. A lot of being able to squat well is all on technique.
You talk about mobility. Have you done mobility WOD "the first of many beatdowns"? It's a 10 minute squat hold and really works on your flexibility. http://www.mobilitywod.com/2010/08/first-of-many-beat-downs.html
edit how long are your rest periods? I grab a bench and sit down for a good 3-5 minutes between each set. I wasn't so sure of this in the beginning, but reading starting strength and t-nation has really sold me on how important rest periods are.
