You create a layer mask in Layer -> Mask -> Add Layer Mask
Layers masks in Gimp are really useful so look in the gimp docs on use
https://docs.gimp.org/en/gimp-dialogs-st...layer-mask
Either the layer mask or the (image) layer can be active, important you know the difference. Click on the icon to make active, a white border indicates it is active.
For your three layers.
Make a black layer mask (full transparency) for each of the top 2 layers.screenshot https://i.imgur.com/1qaFdrw.jpg
In the second layer make a selection. https://i.imgur.com/wDsgBia.jpg
Fill the selection with white. https://i.imgur.com/TWUAExs.jpg
Move up to the next layer, make sure the mask is active, make a smaller selection, fill with white https://i.imgur.com/AS09SPc.jpg
Save your work as a Gimp .xcf file which saves all layers, masks, etc. When happy with the outcome that is the time to Export to a png / jpeg / tiff ...
Layers masks in Gimp are really useful so look in the gimp docs on use
https://docs.gimp.org/en/gimp-dialogs-st...layer-mask
Either the layer mask or the (image) layer can be active, important you know the difference. Click on the icon to make active, a white border indicates it is active.
For your three layers.
Make a black layer mask (full transparency) for each of the top 2 layers.screenshot https://i.imgur.com/1qaFdrw.jpg
In the second layer make a selection. https://i.imgur.com/wDsgBia.jpg
Fill the selection with white. https://i.imgur.com/TWUAExs.jpg
Move up to the next layer, make sure the mask is active, make a smaller selection, fill with white https://i.imgur.com/AS09SPc.jpg
Save your work as a Gimp .xcf file which saves all layers, masks, etc. When happy with the outcome that is the time to Export to a png / jpeg / tiff ...