Theosis requires theology.
Henosis requires a higher power.
Gnosis is just knowledge.
I believe their differences are significant. Theosists (is that a word?) believe on authority instead of first-hand knowledge. Theosis requires a mythology.
Henosis is what I consider a link between Theosis and Gnosis. It often does not have strict mythology. Henosis may have a bit of authority backing it, but often said authority is of the gnosis variety.
Gnosis is as close as one can get to the divine. It's first hand knowledge. No dusty book, no preacher or parent, and no intellectual pondering; gnosis holds no pre-conceived notions.
Personally I view each of these terms as such:
Someone (gnosis) sees something of the divine and tries to tell others about it. (henosis) these people try to link with this, and somewhat see what the first person saw. Then, these people tell others (theosis) where it is told again and again. The divine in this sense is "dressed up" in intricate articles of "clothing" where the clothing is emphasized and the original spark of divinity is often lost in the detail.
I'm feeling cynical at the moment. *shurg*
No offense anyone.