Create a layout file with three panels and use it as the cells layout. The panels should be vertically docked to BOTH sides.
You should use designer script to set the horizontal size and position.
I think Java should have already installed it for B4A. I suppose you meant that you did it in Java, but I find this too unlikely.
In any case, what you are looking for can be obtained with this library: https://www.b4x.com/android/forum/threads/sd-menuextender.88991/