Nope. The riddle says that by adding the two letters, you will have fewer. Five is more than three, so it couldn't work. Also, "I know a word of letters three," translates to everyday English to, "I know a word with three letters."
Few has three letters. Add two letters, and you have the word
fewer which has five letters.
New brain teaser:
A man is talking to the jailer. He says: "Brothers and sisters have I none, but this man's father is my mother's son."
Who is the man's father?
Orange Blossom
Help us help you. If HelpBot replies, you MUST follow step 1 in its reply so we know you need help.
Orange Blossom
An ounce of prevention is worth a pound of cure
SuperAntiSpyware, SpywareBlaster, WinPatrol Plus, ESET Smart Security, Malwarebytes' Anti-Malware, NoScript Firefox ext., Norton noscript