hostmakedepends: All the stuff that the building machine needs to be able to run through the building process. Mostly tools, like pkg-config or somesuch. This is needed regardless of which architecture you are building for.
makedepends: All stuff that is needed du build the package on the building machine. Usually devel-versions of the required libraries. This is dependant on the architecture you are building for.
depends: All the stuff required to run the package on the target system. Not needed at all on the building machine, and dependant on the target architecture.

I just realized I never clarified myself here, my bad.

The way I look at it:

hostmakedepends: Depends that are not architecture specific.
makedepends: Depends that are architecture specific. - So I check if 32-bit packages exist. If they do, I know it's architecture specific.
depends: Same as what you said, which actually helped clarify it a bit for me.