The design you're suggesting is for a freestanding wall, not a retaining wall.
Not sure what you mean by 3 feet rising to 8. Is that sloping perpendicular to or parallel to the wall? If perpendicular, that rise represents a surcharge, and even if the bank is stable now, you will have hydrostatic pressure once there's a wall built which will need to be accounted for and relieved by drainage.
There's not enough detail in your post and in any case specific structural advise is against the forum charter.
edit: for what it's worth, I have walls like this in my garden (that came with the house) and whilst they haven't failed completely they are cracked, and in places I wouldn't have expected. Do it right, do it once.
Ah, OK. Well a rule of thumb is anything under 3ft/1m is a gravity wall, i.e. will hold up against the pressure with its own weight, and anything taller than that needs "engineering".
