Just a thought...planting 3 of either type of these trees would cause a crowding of the canopy after a few years and probably lead to problems. With a yard your size, you're going to creat a completely shaded yard and if that's what you want, fine but just think ahead so you don't cause yourself a problem in the future. Shumards will grow to an average of 60 fee tall by 40 feet wide on the canopy and pistache will grow 40-50 feet tall with a canopy of 30 feet. That's 70 feet of canopy right there!
Lots of trees is great but I've seen far too many people who plant too many too closely together and end up having to pay big bucks after a few years to take one or more out! Either would be a good choice, or maybe one of each, but 3 trees of this size would crowd each other both above ground and below. Less is more, in this case.
Hope this helps!