Probably, I repeat, probably three of the same cuttings were planted in the same pot to get a heavier looking plant sooner for salablility. One of them probably had an underdeveloped root system or other root problem that caused it not to support the top. Chop that sucker off. Probably.

You can always wait, but my experience has been that when one cane fails it almost always doesn't come back. Probably.

hortster.

Hortster is probably right. You can follow the trunks of your crApe myrtle down to where they join (or where they don't) and find out if your crApe started out as three crApes, or, if it just has three branches.

What ever happened to the one trunk, if you take care of the plant you can have multiple trucks it will just take time. The number of trucks is entirely up to you and how you trim he suckers from the plant. We live east of Raleigh NC, and I have seen some that are one or two trucks that go 20 to 30 feet in to the air. I have seen some equally old plants that are 10 to 15 feet with dozen of trunks.

In some ways they are tree like in their growth habits. They grow relatively fast in the first years and then take years to get big.

Horster is probably right.
It's amazing what the nursery business is doing to sell trees and plants today.
What I can't stand is that they also fertilize the trees to death and there are no roots in the pot when you go to plant them.
Personally, I would just cut it down.
It looks awful and its on your front lawn.
It'll either grow back or not.
In any case, don't worry, it'll sucker a replacement up soon enough.