Why is this the answer?

The answer is few because this is a super clever puzzle that plays a trick with words, not with math. It is like a secret code hidden in plain sight! Let us look at the clues one by one. The first part says, "I know a word of letters three." The word we are talking about is **FEW**. If you spell it out, F-E-W, you can count that it has exactly three letters. So, that clue is solved! Now for the tricky part: "Add two, and fewer there will be!" You might think you have to add the number two or the letters T-W-O, but the puzzle is even sneakier than that. The riddle is asking you to think about the word **FEW** and then add *two* new letters to it to make a new word. If you add the letter **E** and the letter **R** to the end of **FEW**, you get a brand new, five-letter word: **FEWER**! Why does that make the riddle true? Because the word **FEWER** means a smaller number or a smaller amount of something. For example, if you eat some cookies, you now have fewer cookies left. So, by creating the word **FEWER**, you have made the riddle right! You added two letters, and the result is a word that means 'less' or 'fewer'. It is a wonderful piece of word magic! The whole riddle is a giant clue telling you to make the word **FEWER**.