Why is the third Shabbos meal called Shalosh Seudos - literally three meals - would it not be more appropiate to call it Seuda Shlishis?
Often times in the winter when it is a short Shabbos afternoon it is difficult to eat Shalosh Seudos as people are still quiet full from the morning meal. However the Friday night meal and Shabbos morning meal people are hungry and have a appetite for them. Therefore when they eat Shalosh Seudos which is only being eaten in honor of Shabbos that shows that the first two Seudos were also in the honor of Shabbos. That is why it is called Shalosh Seudos. (Sefer Divrei Emes)
There is a mitzvah to have "Shalosh Seudot" on Shabbat. Commonly, seudat shlishit is barely a meal in the modern meaning of the word. (you might be insulted if you were invited to one and didn't know what it was.) For that reason, I think people give it the name of fulfilling the obligation of the mitzvah (i.e. a bare minimum). As in,"We are going to fulfill the mitzvah of "Shalosh Seudot", or shortly "We are going to Shalosh seudot, or as its really pronounced now a days, "We are going to shalushudos"
In many Modern Orthodox shuls, you will see it advertised as "Seuda Shlishit"