The third PPH is tricky to find - see the signpost for Bob's exact quote.
You should only try to find PPH3 AFTER:
Oh, and the key to its door is apparently at the bottom of the lake. Perfect!
Unlike normal walkthroughs, in Nice Game Hints you can select just the puzzle you want hints for. Here are listed the puzzles for "The Third PPH" in Dexter Stardust: Adventures in Outer Space. The guide you choose only contains information about that puzzle, reducing possible spoilers for other puzzles.