The first time you press it, it should only create Slot1. Make sure the “Break” is actually called, otherwise the For loop might iterate through the whole indices.
Other than that, i have no idea why it would create all 3 slots. As soon as the loop breaks or ends, it will only create the slot that is saved in “CurrentSlotName”.
Maybe something else is calling your function multiple times?
For displaying it: Create a function similar to the one i posted. Create a for loop from 1 to 3 and check again if the SaveGameExists (With Slot appended to the Index like
in the function above). If it exists, you load the save game, cast it to your custom savegame class and get the PlayerName that you saved earlier.
You can save this to an array or so and fill the Button Text with it.
Or you could bind the text variable to a function with “DoesSaveGameExist (Slot1)” etc and just load the PlayerName for each button text individually. If you don’t know how to work with
savegames, i recommend you to look that up 
Would waste too much time explaining something that has Tutorials up on youtube.