Why does the Torah add the word "Echad" regarding each of the species?
Rashi (from Sifri): To teach us that it was a prize animal (Meyuchad sheb'Edro). 1
Moshav Zekeinim (21): In Shavu'os 15a, it says that [size] is not important to Hashem! (He implies that he questions Rashi, but Rashi did not say that it is superior due to its size! - PF)
What is the significance of a bull, ram and lamb?
Rashi (21, citing R. Moshe ha'Darshen): They correspond to Avraham, Yitzchak and Yakov, respectively (representing Zechus Avos - Ramban (2, from Bamidbar Rabah 13:14). Hadar Zekeinim (19) - Torah was given in their merit. 1