From a running perspective, I would do 7.5 hr days M T Th F.
From a lab perspective I would do 7.5 hr days M T W Th. (Actually, I would do the 6 hrs MTWTh, but I think this option offers a compromise between running and labwork for a kid who still needs to enjoy summers! Nothing wrong with that at all.)
Assuming you mean a biomedical research lab, which I think is what most people mean when they say "lab," ideally you'd be in every day because there are often experiments that take multiple days to complete.
So I'm kind of surprised that they're letting you choose your schedule like this, because somebody who goes in for 10 hrs a day three times a week will be nowhere NEAR as productive as somebody who goes in 6 hours a day 5 days a week. And I can't believe they'd just let you take Wednesdays off too, which would mean that you can't start some experiments on Mondays or Tuesdays, depending on what experiments you'd be doing. And, for that matter, you couldn't start some on Thursday or Friday since you'd be off Saturday and Sunday too.
If you choose your last two schedule options you'd really just be doing somebody else's pipetting, which you can probably spin on a resume to be equivalent to something substantial, but it's not. There's just a gigantic qualitative difference between the contributions you can make intellectually when you're a pipette monkey vs. actually performing your own experiments.
If you really want to learn something, choose one of the first two options. Like I said, I think the second is a good compromise between trying to get the mileage in and actually getting something valuable out of the lab.