-Hold all three units in a vertical orientation (this is a must), but preferably with a somewhat low profile
-Preferably something that doesn't add a lot of/any width, since they're already in a confined space
-Looks nice or, preferably, barely visible
-Available in the U.S. (Etsy has some that are close, but in foreign countries and have delivery dates several months out)
-Sturdy (obviously)
I've currently got a KV Gear stand, but I don't like it for this purpose. It's too tall, too deep, has too much space between the units, not very stable, and it's fugly af. The add-on top tier is pretty flimsy and it wouldn't take much to knock the top unit off and have it smash the other two units on the way down. They're also way overpriced for what amounts to a few pieces of plastic, some spacer bars, and some screws.
I like this design, if only it had a third tier and was made in the U.S.:
