just buy like 2 or 3 saplings and plant them close together. Eventually they will grow together and form a multi stemmed tree. Or,you could buy one and let it grow for a while, then cut it down and it will send up multiple stems.

Yes, they do sprout from a cut off stump.
The only danger is when those three trunks get larger, they can collect more snow or ice than they can handle. Same with Birch.
Mike

Mike - are you making the case for a single stem tree? vs a multi-stem?

Yes. Multi stemmed trees will always have a loser eventually to snow or ice in my climate and I assume in yours too. I've had it happen with Birches, (Betula) Mt. Ash (Sorbus) and Katsuras, (Cercidifolium). I don't do three in a group anymore.
I used to grow Katsuras commercially. Even sold 600 once in one order. Still have about 1/4 acre left. All grown together and in terrible shape. I am in the process of thinning them again. First time I did it was to cut them off at the stump. They promptly sprouted multiple trunks and created a worse mess than before.
Mike
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Cercidiphyllum magnificum