I'm planning to build a few concrete block garden beds, probably
three blocks high...

I would think that mortar would be needed or the whole thing will fall apart.

I built mine TWO blocks high, on carefully leveled soil. No mortar.

It is just the right height to sit on the edge when I weed it. And it
has not fallen apart, after five years. Gravity and friction hold the
blocks in place.

vince norris