No worries.
I just wanted to make sure I was somewhere near before going further.
In short if you know, or at least have tried, Layer Masks then that's half the battle won already.
The trick is to get the layers in an order that makes 'masking' as easy as possible.
With three layers it should be fairly obvious that one will act as a background, and the other two will each have a Layer Mask to show / hide what you want of each layer respectively.
Just as a side note, having the fourth 'empty' image is extremely useful....you never know when you may need it! I didn't need it here but at the expense of a few seconds taking another photo it could prove to be invaluable.
So....its just a case of getting the layers in order and using Layer Masks. I based my Layer order on the order in which you appear in the 'final' image...so left to right equates to top to bottom in the Layer Stack.
This is the layer stack I came up with and the associated Masks for the top two layers...
I think you'll be OK with this if you've had experience with layer Masks before and these are really easy 'paint with either Black or White' masks too.....nothing complex.
You could of course make more precise Layer Masks and there are lots of threads here on the forums about 'Making Selections' from which you can make a mask.
Anyway....I don't think you'll struggle with it, but if you do get stuck just ask.
Regards.
MrToM.