The phrasing of this question is a little tricky. There are a couple ways of interpreting it.
Brian's First Feast Day is also the day that he remembers as the Day of First Meat. This is where he eats his fill of fish. The book then states that he continues eating fish for a period of time wherein he is sprayed by the skunk and fixes his shelter and food storage problems. This lasts for a minimum of three days, since it took him three days to build his shelter.
The second meat that Brian catches is a foolbird. It takes him some time to figure out how to prepare it and cook it properly. There are quite a few references to the chickens that his mother cooked.
The third meat that Brian catches is a rabbit. He skins it and cooks it, but the description is less vivid than that of the bird.
The reason the question is tricky is that Brian does not name these days as "Meat Days." He actually names the day he caught the rabbit as "First Rabbit Day" and the day he shot the arrow straight enough to kill a bird or a fish as "First Arrow Day." Other than that he doesn't really nickname days.
If the question is truly referring to his third day of eating meat, the answer would be fish. Otherwise if it is asking about his third meat, like I think it is, the answer would be rabbit.