3x3x3, conveniently a cubic yard, is just round numbers. Having 6 fewer inches in depth will not break the composting process. If these are unlidded, then you can heap up to the 36" depth, and if they have open bottoms, you can provide a foundation to raise them up.

thanks Larry! I'll consider a foundation--their location on my way too big sloped driveway may make that tricky. I'll get a piece of fiberglass for a lid so I won't do too much mounding, except perhaps in the summer. It's all gonna shrink down anyway. Maybe I need to just give it my best shot and hope that hot compost happens. If not, go to plan B.

If just yard waste & not food waste how about making a circle wire bin using tposts or wood 1x1s? Then you can get it larger & taller. Upside down carpet or cardboard works as a lid. If you're doing food waste you might prefer to use a closed container so you don't attract rats.

The trick being the deeper it is, the harder it can be to keep everything turned or mixed.

Rats are on the rise in Portland; composting activities may be contributing, along with lax security of city roll-cart lid closure and discarded food.

These compost bins will be for yard stuff only--no kitchen. I have worms for that. But then, rats chewed through the bottom of my (old) worm bin and ate the food and most of the worms, but that's another story....