If they're only 4" to 8" tall, they must be toothpick thin. Your first goal is growing a halfway decent trunk. Let them grow for a few years to thicken up the trunk. After a few of growing out and developing a decent trunk, you'll have much more to work with and it'll be easier to decide how to style each tree. There's just more to work with than a 6" tall toothpick thin trunk with a few leaves. The link below is to an article by Brent at Evergreen Gardenworks about developing trunks. It should cover most everything about developing good trunks. Here is a link that might be useful: Brent's article on trunk development

Use the one on the right to make the future top. Nibble at the other two for a few years to suppress them, while letting the one on the right surpass them. When it has become significantly more dominant cut the other two off completely. The one on the left is at present so big, relative to the main trunk diameter that cutting it off all the way right now would live a giant opening in the side of the main stem. If it weren't at so angular it could be used to become the new top, but the angle of the one on the right is much better. Left partly intact for a time the center and left branches will make food to support the growth of the now to be favored future main trunk. Or transplant it into the yard and replace with one of the Tatarian maple selections on the market that has colorful autumn foliage, red samara wings and an erect habit with a single trunk. Even after your Amur maple is trained to have an elevated crown on a single trunk it will still attempt to form a low, broad head that is liable to be unsuitable for a parking strip. For instance, one on a lawn in front of a Seattle residence had an average crown spread of 45' when 24' tall (1993). You have to train a non-dwarf Amur maple to branch way up there for it to fit anywhere there is not a large space for the explosion of nearly horizontal, pokey branches it will produce. To allow for the passage of trucks underneath a street tree needs to branch some yards above the level of the pavement. A small, bushy tree like Amur maple is going to be tough to train that high.

Sorry, okay, I don't know what all the Maple weeds are, no wonder those have grown so fast! Those are the biggest by far. I'll pull them. I have many smaller ones, they don't have the 3-5 leaflets, maybe because they are small. Last autumn a number of the Maple seedling weeds around my vegetable garden had real nice red/pink fall color, thought most were Amur.

You can space the maples as close together as you want. They will grow. However, for screening purposes, you probably reach a point of diminishing returns. The closer they are together, the more they'll compete with each other, and also grow strangely. I would think than anything closer than 4' apart is wasteful, and probably anything less than 6' together is unnecessary. All that said, I really like the suggestion of the column form of hornbeam. These make great screens and I've never even seen one set fruit, let alone spread invasively. And they'll grow anywhere that Acer ginnala will. If the site is perpetually damp, however, skip either Acer ginnala and Carpinus betula and go right to river birch, which is also a dense, twiggy and a decent deciduous screener. Oh, and regarding Acer ginnala, fall color is highly variable on this one. Many seedlings fail to show any decent fall color. Therefore, it is far better to seek out proven cultivars such as "Flame," which will produce known results. Scott