What is a 3 Act Task?
3 Act Tasks are challenges that are given in three distinct components, that are designed to engage students and focus on problem determining and solving. These tasks are usually given to the class as a whole group or split into small groups. 3 Act Tasks were designed and created by Dan Meyer.
What is the Structure of a 3 Act Task? 
As indicated by the name, 3 Act Tasks are comprised of 3 Acts:
ACT 1: Engage the students and determine the problem. 
In this step teachers present to students a video, image, or other situation to hook students. The video should end with a cliffhanger or an unknown solution.
Students are asked to discuss “what they notice and wonder?”
The goal is to generate questions about the given situation.
Students benefit from hearing questions others generate and by making estimates about what the solution might be.
ACT 2: Seek and Provide Information
Students then gather information that they will need to solve the problems. This might be specific measurements that need to be taken, or perhaps introducing mathematical strategies that have yet to be learned. It is best if teachers offer information to students as they ask for or require more information.
This step is where an agreed upon question that students have discovered/asked becomes the focus of the task. Students are looking for information about the situation that they need to solve the agreed upon question.
ACT 3: Reveal the Solution and Extend the Task
In this final act, students share their thinking, work, and solutions. From these shared solutions, students build towards solutions that make sense and are accurate. The goal of this act is to not just reveal the answer and move on, but instead to build towards students knowing and trusting that their solution makes sense and is justifiable.
This is a point where: 
Students should compare and evaluate their own and others’ solutions.
Students should discuss and understand solutions that are different than theirs
Teachers should help connect students work to the curricular content.
Once students have had this chance to test their thinking, the solution or answer from the teacher comes in support of their thinking. (This is the point where the rest of the video is played!)
Where can I find 3 Act Tasks?
Lots of places! Here are some places to start:
Can I see a 3 Act Task in Action?
Of course!
Below is a link to a modeled lesson by 3 Act Task creator Dan Meyer. This was a session that was run in Cambridge for a collection of Math teachers.
The first link is the entire lesson that was run with these teachers - it is important to remember that this was run in a university session and not for high school students. The delivery presented might need to be tweaked for high school students but the facilitation and pedagogy and “teacher moves” will remain similar.
The second link is a blog post by Dan Meyer breaking down how he facilitates a 3 Act Task (a long post but worth the time).