You can't keep three fields in Mapping Load Table...
Consider that you have three fields
Matching or Lookup fields = Field1, Field2
Result Field = Field3
Try to combine as Below
Now in your main table use as below
ApplyMap('TempMapping',YourField1&YourField2,'Unknown') as YourFieldName3
Hope this helps..
If you want a bit more information on how to use ApplyMap, have a look at this blog:
It will help explain how to construct an ApplyMap and when to use it.