I figure that there are really two unique strats for dealing with the middle position in a 3 v 3, boom for iron or, in a water map such as med or coastal, take control of the water.
My personal preference in water maps is to take control of the water. By this I mean have 7-8 scout boats and 12-15 villys on wood by the 12 min mark. At this point its strictly destroy all enemy docks and fishing boats. Of course this is easiest in the med map and I can't ever remember losing a multiplayer game (I've been blessed with decent teamates on these maps and also at least 3 rounds of the middle position) on med when the water was controlled during mins 10-30. Of course this is a bit harder on coastal and certainly wouldn't work too well on continental (tough to really have a "middle" position).
My rationale for this strat is pretty simple, your teammates can fish for food unmolested and will be in a better position to use their wood for land units. Lastly, even with this strat your bronze time rarely suffers past 18-20 mins (unless someone else really wants the water also) and nothing beats unmolested fishing.
Any thoughts?