The reason for R' Eliezer is because just as in you're house you have 14 meals over 7 days ( 1 by day & 1 by night) so too in the succah you have to have 14. But every 7 days ther's shabbos and on shabbos you need 3 meals?
Manes Blumenfrucht, brooklyn, NY
The Cheishek Shlomo and others answer that Rebbi Eliezer specifically said "Achas ba'Yom v'Achas ba'Laylah" -- "one in the day and one at night," instead of starting off with the first Seudah which is at night "Achas ba'Laylah v'Achas ba'Yom" -- "one at night and one during the day," because he was not including the first Seudah at night which everyone agrees is a Chiyuv. How does he get fourteen other meals? He includes Shalosh Seu'dos.
All the best,
I have seen elsewhere (for example about meals from the Mon in the Midbar) that if one extends Shalosh Seu'dos after nightfall, one can also be Yotzei Melave Malka. Maybe this is why he does not need another meal?
Meir Eliezer Bergman
I don't know if that would be a great answer, as then one could say that he could combine his other meals during the week as well, starting one by the end of the day and continuing into the beginning of the night, as well, without having to have fourteen meals (this would apparently not be a special Halachah regarding the meal of Shalosh Seudos and Melavah Malkah).