There is a node called “Does SaveGame Exist”. Name your slots something like “Slot1” “Slot2” “Slot3”.
If the player enters a new name, use a for loop from 1 to 3 and check with “Does SaveGame Exist” if the slot is already used.
Use the Index of the For loop, together with an “Append” node to do this “SlotIndex” which leads to “Slot1”, based on the current index.
If the SaveGameDoes not exist, save it and break the for loop. There are for loops with a “Break” input. If you call “Break” it will stop.