Well, you can have one big zone for forward taking middle part and for backward, you can set lower part, but it would go across whole screen (not in 3 and 2 separate zones).
But, parts you don't touch, want be important to you.
To do that, you need to edit defaults.lua (rename it afterward to defaults.persistent.lua, so it doesn't get overwritten) in kobo root directory.
Code:
-- customizable tap zones(rectangles)
-- x: x coordinate of top left corner in proportion of screen width
-- y: y coordinate of top left corner in proportion of screen height
-- w: width of tap zone in proportion of screen width
-- h: height of tap zone in proportion of screen height
DTAP_ZONE_MENU = {x = 1/8, y = 0, w = 3/4, h = 1/12}
DTAP_ZONE_CONFIG = {x = 0, y = 0, w = 0, h = 0}
DTAP_ZONE_MINIBAR = {x = 0.1, y = 0.97, w = 0.9, h = 0.03}
DTAP_ZONE_FORWARD = {x = 1/12, y = 0, w = 11/12, h = 1}
DTAP_ZONE_BACKWARD = {x = 0, y = 0, w = 1/12, h = 1}
Edit what is red to suite your needs. As you can see (this is how I set it for myself) you also use decimal numbers, not only fractions.
And for menu, it is either swipe down from top of screen, or can be some other gesture (now we have gesture manager).
Anyway, I think you can't get exactly as you want, but you can get close enough.