If I understand your description correctly, you have 3 poses included in each frame and 8 frames, that means your spritesheet will effectively contain 8 sprites (each one just happens to have 3 characters in it). These are not three separate 8-frame animations but just one, so the number 3 makes no sense in the context of this spritesheet. It sounds like what you want is either a single column or a single row, not 3 columns.
384 is 48 (frame width) x 8 (number of columns), calculated automatically from the settings above.
The power-of-two (POT) presets are there for convenience for use with “Best fit for texture”, as POT-sized textures usually yield better performance. You do not need to edit these numbers