Three Ways?
I'd split them up this way:
One gender lays unfertilized eggs.
The other gender fertilizes the eggs by ejaculating on them (this is how seahorses procreate, I think... Some underwater creature does this)
The third gender picks up the fertilized eggs and gestates them, maybe in a pouch like a marsupial.
Of course, this would all be very impersonal and maybe isn't what you're looking for. One thing's for certain, it would make for some really unpleasant to look at porn.
AJL