Seudah Shlishis has a specific time it should be done, which is roughly Shabbos afternoon (preferably after praying Mincha, but can even be eaten after the time for Mincha has arrived.) See Shulhan Aruch, 291, 2. The SA in OC 289 has a whole section on the laws of the Shabbos morning meal. The fact that the rule of Kiddush Bimakom Seudah still applies for this seudah clearly indicates a time frame for the day meal on Shabbos as being in the morning. I do not see how one is to avoid the obligation of two seudos on the day of Shabbos, lechatehilah.