It's been four years since I did the three gorges trip on Viking, but at that time wine and beer were available at dinner, no charge. Many lunches and several dinners were provided off-ship, at one of the sites visited, and again wine was served, as well as a VERY potent Chinese cordial (?) at one lunch. I never looked into getting wine at any other time.
My trip was in September; the various cities had shirt-sleeve weather, although it drizzled all of the day we visited Tiannamin Square and the Imperial City. In the three gorges area, though, the steepness of the hillsides causes pretty strong overcast over the river, and it can get a little chilly.