Well normally a 3 season deck is exactly that not used in the winter .

Did you ever find someone for your project? Looking for the same thing!

I think in the PNW the 3 seasons are fall, winter, and spring. Our summers are so dry now we don't need covering but we would like to enjoy our not so cold winters outside. We have a friend in the Willamette Valley with a covered deck that isn't attached to the house. We sometimes gather there and another friend brings their propane fire pit. It isn't ideal because the sides of the deck are open. This arrangement probably works better for smaller groups so that all can gather closer to the fire pit. Because it is so open, there is no problem with mildew.