Your code is using a string variable called MyText so we can use a string function to replace a string with another string inside that variable
Code:
Sub sss()
Dim MyText As String
MyText = Selection.Text
Selection.Style = ActiveDocument.Styles("MyStyle")
Debug.Print MyText
MyText = Replace(MyText, """", "/""")
Debug.Print MyText
Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, Text:="TC """ & MyText & """ \l 3 ", PreserveFormatting:=False
End Sub