Napa Valley offers visitors a long list of restaurants, wineries, spas, and other attractions. Given all of the wonderful choices available, I highly suggest that you have an agenda for each and every trip you take to the valley. When planning a day trip that is primarily focused on wine tasting I suggest going to one winery before lunch. Typically, I schedule my first wine tasting around 10:30 or 11:00 in the morning. This allows for a relaxing start after a reasonable breakfast which includes a good cup of coffee.
After the first tasting, I typically have a reservation at a restaurant that is not too far from my after lunch wine tasting appointment. I typically don't eat a big lunch and most of the time avoid having wine with my lunch as I know I will be sipping on several glasses of wine the rest of the afternoon. After a leisurely lunch, I will head of to my first afternoon tasting which I usually set for around 2pm. Given that typical non-rushed tasting lasts about an hour, I wouldn't suggest making your third appoint until 3:30 or 4:00 depending on how far the next winery is from your second stop.
When selecting a winery to visit I typically focus my decisions around the quality of wine being offered. I have received some of my best wine through wine gift baskets. I am not interested in average wine, rather I focus on the top flight offerings that you find in the boutique artisan wineries. These types of wineries are often not household name, rather they are known by the avid collector or wine enthusiasts. Production on the wines being offered at these types of establishments are very limited and may only be sold to their mailing list clients. Often I will sign up to receive one of their wine gift baskets around the holidays.