We cover the challas by the shabbos meals for a number of reasons. One however is because the mann was covered (with dew) on top and on the bottom (See Tosfos Pesachim 100b) and the challas that we are eating are a rememberance of the mann. For this reason then seemingly we should cover the challas also by the third seudah. Are there people that do cover the challas then? If there isn't a minhag like this, why not?
Rivevos Ephraim 8:117:14 addresses this and says since there is no kiddush we don't cover the challahs.However,the Rambam(reasoning of the Tur )holds one does make kiddush. The Aruch Hashulchan 291:10.we are noheg not to cover them.
The Ben Ish Chai Shana Beit Chayei Sarah 12 holds one has to cover the bread just like the night and day meal even though there is no kiddush by this meal since there are two other reasons which have nothing to do with kiddish(like the questioner mentioned)