If you store references to the 3 enemies you can just store them in an array, then generate a random int between 0 and 2 and access that enemy in the array.
Or you could use a ‘find all objects of class’ node set to your enemy type.
OR if they’re different enemies I would recommend that they all inherit from the same base class to make the above suggestion work.
there is another option, but I don’t think it’s as elegant in this case as inheritance - they could all contain a common blueprint interface to let you act on them in the same way. And this is actually how i’d recommend you implement damage. They all have an interface that allows the aggressor to access them and reduce their health.
You can even do things like find all characters that implement said interface.
Having said all this I think there may be some stuff already in UE4 specifically to deal with damage, i’v come across a few things while browsing the list of nodes. It may be worth a google first.