If you click on the "Town" building, this is where you should see your current formation and be able to change it. Teams are made up of groups of up to three heroes. The point here is to think about team composition. Some heroes are primarily PvE-oriented but that doesn't mean they can't be used in PvP, and vice-versa.

Always make sure to check your formation before doing Adventure Missions, Mines, Arena, or a boss. Change your heroes to fit accordingly.

https://lh3.googleusercontent.com/AGPIdatTtric7gU-Kjtu1mLLAlnW8BnqnwtpfhxyV6jF5KgEz3yT6b85KS6erU85t_XI2QbqzHwA2nEJsKXQG37x5BfDbgFVWzFUkKr119KzNi1w26NywmeaMPEAPrj2FOa9cQOHNx0Y40LryQ=s0