The mate/harpooner pairings are as follows: Starbuck's harpooner is Queequeg, the south sea islander Ishmael shares a room with in New Bedford. Stubb's harpooner is Tashtego, the Native American. Flask's harpooner is Daggoo, the African.
The three mates are very different in temperament. Starbuck is a veteran whaler and a hard-headed rationalist. His interest is in a profitable voyage, and Ahab's thirst for vengeance is at odds with his business sense. However, Starbuck's sense of duty prevents him from trying to stop Ahab. Stubb also is an experienced whaler, but unlike Starbuck, he has a keen sense of humor and loves to make jokes. Flask, the youngest of the three mates, is the most excitable.
The multicultural origins of the three harpooners are significant as well. Tashtego, one of the last of his tribe, is a skilled whaler and an essential member of the crew. Queequeg was a prince in Polynesia; Daggoo similarly is described as noble. One possible way to think of these three are as types of the "noble savage." It's also important to consider the three in relation to the problem of race in America; while the harpooners are clearly skilled, they nevertheless come under the command of the ship's white officers. So, while on the one hand the Pequod's crew represents a kind of "brotherhood of man," on the other it can be seen as an expression of white supremacy.