In many language, adjectives can and sometimes usually preceed the noun they refer to. “Third” is the adjective. What is pronounced as “Shalosh Seudos” is Yiddish, even though the words are of Hebrew derivation. While this sounds weird to a Hebrew speaker who anticipates Semitic syntax (with the noun prceeding the adjective), it sounds quite reasonable to native speakers of Yiddish and English. (whose adjectives come first).
This is an interesting linguistic topic with no deep halachic or phiosophical meaning.