After you input your age, on the screen with the first question, use the following key combination to skip the quiz: Alt+Ctrl+X. The game will then let you pick the level of "filth".
The GOG.com re-release provides the correct answers in <path-to-game>\LSL3 Answers.txt.
A list of codes is provided by Al Lowe on his Humor Site.
The GOG.com re-release provides a copy of the game manual in <path-to-game>\Leisure Suit Larry 3 - Manual.pdf.
<path-to-game>\Leisure Suit Larry 3 - Manual.pdf