I guess you want to add to selection? You can't do that on the 3rd empty layer.
Switch to your first layer, and use alpha to selection.
- Alpha-To-Selection-Add1.jpg (201.75 KiB) Viewed 1516 times
Switch to the 2nd layer, and use add to selection.
- Alpha-To-Selection-Add2.jpg (319.55 KiB) Viewed 1516 times
Now you may use the combined selection on the 3rd layer if you wish.