This instructional routine is intended to support students in using mathematical modelling to solve contextual problems and to learn how to have mathematical discussions with each other. It was created by Dan Meyer.
In the routine students are prompted to develop a mathematical question, generally through the use of a video or photo to pique their mathematical curiosity. Students are asked to provide a “just right” guess as to the solution, then determine what they need to know to solve the problem. Students then work to create a solution to the problem and finally the answer is revealed, again through the use of multimedia.
Here is a video of Dan Meyer leading a Three Act Math task in a conference setting.
The flow of the instructional routine is
Orient to the task: 

Tell students why they are doing this activity and how the activity will run. 
Develop a Question: 

Students look at an example of multimedia, share their possible questions with a partner. 
Share Questions: 

Selected pairs of students share their questions with the whole class. Students are then prompted to consider an answer that is too low, an answer that is too high, and a “just right” answer to one of the questions. 
Model the Problem: 

Students develop a model for the problem, asking for more information if they feel they need it while the teacher circulates around the room. 
Share Solutions: 

Selected students share their solutions to the problem and then the answer is revealed. 
Metareflection: 

Students write independently, students share what they wrote with a partner, then selected students share their reflections with the whole class. 
Learn more about the routine: 
Resources: 
Adapted from Dan Meyer’s blog and Teacher Education by Design: https://tedd.org/threeacttasks/
Here is a list of additional resources for the Three Act Math tasks available via WCLN. The template for the slides was produced by New Visions for Public Schools.
 Apple Mothership [ task  source  slides ]
 Bet Your Coffee [ task  source  slides ]
 Car Caravan [ task  source  slides ]
 CircleSquare [ task  source  slides ]
 Coca Cola Pool [ task  source  slides ]
 Coin Counting [ task  source  slides ]
 Ditch Digger [ task  source  slides ]
 Domino Skyscraper [ task  source  slides ]
 Double Sunglasses [ task  source  slides ]
 Falling Glowsticks [ task  source  slides ]
 Ferris Wheel [ task  source  slides ]
 Fry's Bank [ task  source  slides ]
 Hot Coffee [ task  source  slides ]
 Lucky Cow [ task  source  slides ]
 Meatballs [ task  source  slides ]
 Money Duck [ task  source  slides ]
 Nana's Paint Mixup [ task  source  slides ]
 Obscure Geometry [ task  source  slides ]
 Penny Circle [ task  source  slides ]
 Pixel Pattern [ task  source  slides ]
 Playing Catch Up [ task  source  slides ]
 Pool Bounce [ task  source  slides ]
 Pyramid of Pennies [ task  source  slides ]
 Rotonda West, FL [ task  source  slides ]
 Scrambler [ task  source  slides ]
 Shipping Routes [ task  source  slides ]
 Super Stairs [ task  source  slides ]
 Taco Cart [ task  source  slides ]
 Thanksgiving Taters [ task  source  slides ]
 The Incredible Shrinking Dollar [ task  source  slides ]
 Ticket to Ride [ task  source  slides ]
 Toothpicks [ task  source  slides ]
 Water Tank [ task  source  slides ]
 Will It Hit The Hoop [ task  source  slides ]
 World Record Air Bag [ task  source  slides ]
 World Record Balloon Dog [ task  source  slides ]
 You Pour, I Choose [ task  source  slides ]