I am planning straight sides of equal length; is that a mistake? If not, I don't see why pleats are needed.
I think the three inches you refer to is the 78" body versus 81" to include seam allowances.
I usually make the bottom layer of my insulated hammocks about 6" longer than the hammock bed because there's a lengthwise curve in addition to the side to side curve you have allowed for. You should probably solicit advice from people with more specific underquilt experience than I have, though.