not sure
I think this would be interesting just as a curiosity as far as boot configuration. I don't see why it wouldn't be possible. You would just install it the same as if it was real hardware.
But from a practical standpoint, aside from having one .hdd file as opposed to 3, i don't think there is any advantage. If you use expanding drives, the amount of space you use should technically be the same.
Having separate virtual machines, on the other hand, would give you some advantages of being able to boot at the same time, suspend one machine, etc.