idea:
- they are three of the same kind (sisters/brothers)
- usually, fish and trees don't meet
-

is "poor": only 2.5k points
-

is red - and when someone is shy, he/she is red too
-

is the "King fish" and it is yellow/golden like a crown; so it is royal.
So the riddle's answer is
I only can't figure out why

is adventurous. Is it because it is so common? Or does the blue colour mean something I don't thonk of?