Do you think placing the Poplar the same distance from the side and back of the yard as the Elm, but on the opposite side of the yard will be enough repetition?

Yes. I only brought up that concept (of repetition within the landscape) for something for you to consider going forward. I'd not change anything about your current plan, but would, in the future, use some other type of plant of landscape element to tie things together. The 3 tree types you've selected are just fine.

+oM

Another option would be to center the one tree in the far back and then plant the two on the sides closer to the house. (If moving the elm is not an option, ignore!). That way you will be able to see all three trees more or less from the center of the yard. You want to think in terms of sight-lines from key windows or patio. Not knowing more detail, my first instinct would be to center the oak in the way back and the elm and tulip tree on the sides because they will likely wind up a bit narrower. Also, if you are up for mail order, you may consider a newer variety of tulip tree that has been bred to have the strong central leader and a tighter form. Either way, if you plant a big shade tree in the middle of the yard, the two in the back corners will be tougher to see. Just some thoughts. I have been wrestling with the same questions you are about located backyard shade trees.

Pick your red oak and tulip (liriondren I suppose) in the fall for the best chances of good fall color. All trees I like btw.

We aren't going to move the elm tree, and I also don't want to plant the Northern Red Oak close to the property line in case neighbors don't appreciate the acorns, so more center and in front of the yellow fall trees will ensure that the oak doesn't grow past the property line. I THINK that since the other two trees would both be near the left and right edges towards the back, that they won't be fully blocked in view. I am also thinking of not planting the Oak dead center, and instead a little right of center with future plans to plant a very small tree in view of a window that is left of center.

We also don't have the option to pick with fall color, it's a town sale where they are selling 3 to 5' trees in early September, so I doubt color would change by then.

I appreciate all of your feed back that you think the trees would work well together! As for tying it together in the future, what exactly do you mean? I am quite new to this, and this whole not planting everything that is the same in a perfectly straight line thing is against my nature :D.

Thanks!

Regarding the property line, I think you will be OK if you plant the oak 20' or so in from the line, assuming there are no obstructions from the neighbors' yards. In 50 years, you may wind up with 5 or 10' of overhang on the neighbors property, but that is to be expected with shade trees and you and your neighbors will all be long gone. When you look at the mature neighborhoods with the really old shade trees, you always see trees overhanging property lines. I think it is unavoidable if you want to plant multiple shade trees, unless you have a huge property.

Jen, I fear I may have brought an unnecessary concept into this discussion, re: Repetition in landscape design. All I'm saying there is that good, harmonious-looking landscape design often has repeating elements. But you can completely disregard this for what you are doing here-planting three trees. And some very nice trees at that!

+oM