The leaves look like plum to me (but that doesn't guarantee they are plums, it only means there isn't an obvious reason why they are not plums!)
They are still very small and look like they might have been grown from seed. They certainly don't look as if they were bought grafted onto a rootstock.
If that is correct, they may take up to 6 years from germination to getting the first crop of flowers and fruit. From their size I would guess they are 2 or 3 years old now.
If they were grown from seed there is no guarantee they will grow true to the original variety, so the only way to tell what you have is wait and see.
Next spring, I would think about trying to decide which of the branches will make the eventual shape of the tree, and prune the rest. You don't want to keep branches that are crossing each other and may rub against each other, for example.
Also they are much too close together for the eventual size of the trees, and the best time to move them is when they are still small. Mature plum trees are typically pruned to about 15 feet high and a similar diameter, and left unpruned they will grow to twice that size.