If you look around through destiny stuff 3 is a recurring number.
Ex: Seven seraph logo has three.. whatevers.
FWC logo has three main colors/the way the f is set up
dead orbit symbol center thing has three lines under it
Monarchy symbol has three lines in it
Destiny logo has three sides
Three classes
The logo's probably represent the size of the fireteam, though.
Edit: other reasons:
Three forms a triangle, something strong and hard to break.
English
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This. 3 classes, 3 races, 3 points in the tricorn (destiny logo), 3 in a Fireteam, 3 carryable weapon slots. I do agree that the tricorn is probably just symbolizes a three man guardian Fireteam protecting the traveler at its center.