Talk to the witch. The third drug needed in the forest is called "Smurfette drug". Ask about the Smurfette drug.
This is the low-spoiler guide for "Smurfette drug" puzzle in Lancelot's Hangover. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.