This will give you all the values selected separated by a comma:
If(!IsEmpty( cboCombo.SelectedItems), Left(Concat(cboCombo.SelectedItems,Title &", "),Len(Concat(cboCombo.SelectedItems,Title &", "))-2) ,"")
If you haven't selected anything (combo is empty) you get an empty string, otherwise each value selected is concatenated with a ", " separator. The last 2 characters are trimmed off so you don't get a trailing ", ".