If every meal on shabbos is a seudas mitzvah why does sudas shilshis not have the honor of having a kiddush?
According to the Tur's OC 291 reading of the Rambam Shabbas 30:9 one should recite kiddush by the third meal and so did the Shibulei Haleket's Rebbi(Rav Meir Bar Moshe). See the Kesef Hamishna 4 who disagrees with such a reading and brings many opinions. He adds that one who is makdish does not lose anything.
The Shibulei Haleket explains why we don't say kiddush by the third meal in Siman 93(don't have access now to it to read it)