This is a bit technical but there are multiple versions of HomePlug (it is a standard).
Homeplug AV2 does support multiple phases. I have a set of these:
http://www.takealot.com/tp-link-av1200-3-port-gigabit-powerline-adapter-kit/PLID38533225
Although I don't typically run it on different phases I have verified it works.
Homeplug AV2 uses the neutral, phase and ground conductors. So you should actually have all 3 connected for optimal performance.
Since 3 phase shares a neutral and ground conductor, it is should be obvious that decent speeds are still very possible across multiple phases.
When I used the version above I rewired it to work with a normal SA plug so that all 3 conductors are connected correctly.
For Homeplug AV2 polarity is not sensitive. Upon startup it will determine which conductors are suitable.
It is worth mentioning that they recommend plugging devices AFTER the Homeplug (so plug it INTO the Homeplug). Especially high noise devices like Plasma TVs. They reduce the noise on the line with some smart circuitry.
Ellies surge plugs make these things not work. This I have confirmed.
Other than that, these things work incredible well. I'm actually getting around 400mbps with the TPLink that I linked above due to all 3 conductors being connected.
Well worth it, performance is much, much better than wireless!