I would think that since they have been growing together in multitrunked form already, trying to separate them now would likely cause more harm than good even if they started out as three separate trees. It isn't as though you are trying to put three separate trees in one hole.

It isn't as though you are trying to put three separate trees in one hole.

Actually that's exactly what it is, even if the separate trunks actually belong to a single specimen - unless planted inside a grove of birches, where side shading from other trees will keep them growing toward the sun the trunks will lean away from one another as the clump ages. This produces an appearance of conflict or falling apart over time.

However I wouldn't try to separate deciduous trees in June. As it is you may have bought one(s) that were in poor or stressed condition to begin with, if they were on special.

They usually are three separate trees...the typical way of doing this is to stick three saplings in the same pot. This looks awesome when the tree is young. No idea what it does in the long run, but probably doesn't maximize the trees life expectancy. I've seen old single specimens of River Birch...they look fine. They might look kind of sad and lonely when the tree is small.

If you ARE going to separate them, I'd do it now. Not the ideal time of year, but the alternative is planting them now, digging them up later, and traumatizing them twice. You may want to pull apart circling roots anyway on a clearance tree. If they are potted them out of the pot and them in a bucket of water briefly to soften the roots and make it easier to pull them apart. Then I dump the bucket of water th the planting hole(s?) and wait for it to drain. Maybe partially shade them with a tarp when the real heat hit?

Digging them back up later when leafless or nearly so would not cause them to shrivel, be damaging them right before hot summer weather comes. Optimum timing would probably be at the end of winter, at the same time of year that new bare-rooted stock is bought and planted. Spring bud break is the time when new roots grow from cut or damaged root tips.

You'd just have to be sure to get around to digging the specimen back up next year.

Thanks for all the replies!

Apparently I wasn't clear. I realize that there may be three trees growing in the one pot, but they have been growing together not separately, hence not three separate trees. I am picturing trying to remove and separate the totally entangled roots of three 6' trees that have been growing together, and can't honestly say that I think it will be successful. I do know that in the tree fields of the defunct tree nursery next door there are "multistemmed" birches that are 25+ years old and still growing fine, undamaged by heavy snow and ice and apparently stable. I think I would be more likely to either return the tree(s) or cut off two trunks and plant the remaining one as upright as possible, trunk wise.

The way I look at it, it's better to have a living multi-trunked tree than 3 dead trees.