Make a Calculation field with text type.
Left ( UserName ; Position ( UserName ; "/" ; 1 ; 1 ) -1 )
But it would be better design to put these three names in separate fields in your database. Then no special expression is needed to access just one of the text items in this field. If this is how the data is imported from another source, I'd use expressions such as that shared by Phil_1986 to parse the data into separate fields as part of the import process.