The answer to the riddle "What is too much for one, enough for two but not enough for three?" is "a secret." The riddle refers to the fact that when one person has a secret, he is desperate to tell someone about it; but when three people know it, it is no longer a secret.
As Benjamin Franklin said in Poor Richard's Almanack, "Three may keep a secret, if two of them are dead." According to the Encyclopedia Britannica, a riddle is a deliberately enigmatic or ambiguous question that requires a thoughtful and usually witty answer. Scholars generally recognize two main forms of riddle: the shrewd or witty question and the descriptive riddle, which usually describes an animal or object in an intentionally enigmatic way.