Let me try to help you with this--I also like black locust trees.

I would do as little pruning of this tree as possible. If it is spreading out and that is a problem, cut it back so it is out of your way. If there is a main stem that seems to be growing upright and it forks, wait another year and cut off the weaker half.

This tree on its own will develop a good upright form (unless by some chance the sprout is a strange mutation)--it will just take a little time for it to develop its good natural upright tree shape. As it grows you can gradually prune off the lower limbs to mach your other one. Patience! If you don't prune too much too soon the tree will grow strong more quickly

--Spruce

I would agree with Spruce on this - wait and see. The more leaves the tree has, the more it can feed the roots - despite common thought, without good roots below ground, you won't have any good tree above ground, so a really good tree has really good roots! Of course if any branches are in the way now, you can certainly shorten the most obtrusive one(s) back to where they are out of the way. You can always prune them off another year. As a general rule of thumb, don't take off more than 20-25% of a tree's canopy/leaves in any one year.

If, by the end of the summer, none of the 3 branches seem to want to become a strong leader and grow upward, you can get as long a stake/cane as you can - 6-8' above ground, 2' or more below - drive it in next to the "new" tree, best done in moist ground. Try to do this on the side away from the "parent" trees, so you don't disturb the runner/root if there is one, since you aren't sure if this is a seedling or a sprout from a runner - and loosely tie the most up-right-growing, or the strongest, branch to it, so it points up, for the most part - you can ignore the very end - but be careful not to put so much pressure on a bend that it snaps or cracks. Tie it tightly enough it will stay there, and tie it with something thick and soft enough that it won't bind or rub, but let it move a bit, so it gets stronger as it grows. Sometimes, if a tree is staked tightly, to grow tall very fast, the trunk doesn't get thicker, and the tree flops over when untied - not a good idea, esp. since you are trying to get away from flopping. You also want to be sure that the ties aren't so snug that they cut into the bark - you probably ought to check once a month or so to be sure they aren't too loose, too tight, nor rubbing. Check in the late spring next year if you can untie it or not - see if it flops when untied - if the stake is still needed, tie it up again, and check at the end of the summer. Untie it lower down, and move the ties up the stake, as it grows. You want the most of the supports to be above the point where the tree flops as you hold it vertically and move your hand up or down the trunk - don't worry too much about a slight floppiness in the very top, thinner section - that should hold itself up as it grows.

And all this may be totally unnecessary anyway. Most trees WANT to grow upward, and even if they droop a bit at the ends of branches, will hold themselves up as they grow more. There are weeping cultivars that NEED to be staked to get any height, but I don't think there is a locust that does that. And some trees do need a little encouragement to be tall and straight when they are young....