It's hard to distinguish the green "native" to the tree and the green from the grass so
Color erase is going to make things a bit transparent all over.
- Put the top layer in "Difference" mode.
- In the Channels list, drag the Green channel to the general area to make a copy of it
- Make a copy of the three+grass
- Add a layer mask, initialize to the copied channel
Color > Invert the mask
- Threshold the mask (very low value)
If needed you can paint the mask to restore pixels.
(the layer group isn't absolutely necessary, it is only used to show what the Difference looks like)
Edit: a slighty simpler/faster workflow is to replace steps 2 & 4 by:
- In 2., right-click the Green channel and
Channel to selection
- In 4., initialize the mask with the selection.
Also, make sure that you aren't de-selecting the Green channel in the process (which is what happened in the screenshot above). You may have to explicitly re-select it.