I don't actually know for sure, I would think if dinner didn't start until 3:30, then a 3:00 reservation is lunch. They might put out bigger dinner items in that half hour period, but I still think you're going to be charged for lunch.
Lunch 12:00 - 3:45pm, Dinner 4:00-8:30pm
Oktoberfest Adult Lunch Buffet - Lunch to 3:45pm - Seasonal salads, Traditional sausages, Rotisserie chicken, Roasted pork, Red cabbage sauerkraut, Potatoes, Spaetzle, Schnitzel, Frikadellen, and Assorted Desserts including Apple Strudel, Black Forest cake and Bavarian Cheesecake. - $19.99 *
Oktoberfest Adult Dinner Buffet - From 4pm - Seasonal salads, Traditional sausages, Sauerbraten, Rotisserie chicken, Roasted pork, Fish, Red cabbage sauerkraut, Potatoes, Spaetzle, Schnitzel, and Assorted Desserts including Apple Strudel, Black Forest cake and Bavarian Cheesecake. - $28.99 *
If you want sauerbraten, you have to go for dinner.
http://www.wdwinfo.com/wdwinfo/dining/diningdetail.cfm?restaurant.ID=102