Little windmill, say it ain't so...
Story-related, cannot be missed.
In Episode 3, once you arrive at the professor's home, you will need to do an easy puzzle to get his package to the roof. Upon completion of this task the professor will ask you to help him out with another task before he will talk with you. While this task is not a hard one, a few of the puzzles will require some good mathematical skills.
Your job is to help the professor fix all the windmills that are scattered across the rooftops. Evelyn will offer to help you out with this task to help get it done.
Cross the rope to the left to get to the next rooftop. Follow the rooftop edges to the North to get to the first windmill. Here you will need to connect connectors so that the number you start with ends up being the number in the middle and then again at the end. To do so you'll need to add and subtract the numbers on the connectors to get to the desired sum.
Windmill 1 solution:
This will give you a cutscene showing a bridge to the North lowering. Head around the roof to the West then back North to reach the next windmill.
Windmill 2 solution:
Now where the birds were on the roof is a newly-opened path for you to jump across to the next rooftop. Use the rope on this roof to cross to the roof to the South (the lower rope is not needed.). From here use the gutter to the East to shimmy to the roof to the South. Evelyn will now call to you for some help. Cross the rooftops towards her and wait for her to lower the ladder. Climb up and help her solve the final windmill.
Windmill 3 solution:
A cutscene will now play showing you have all the windmills going, and the trophy is yours.