After you select the Formula tab you should see this:
max(0, ( film.toNumber() + exercise.toNumber() + build.toNumber()) / 3)
Write the name of your task in front of the .toNumber() and it should appear on the property
Click on the property of your icon name
Add all the tasks you added

On the last task add a extra ) like shown above, before dividing by the number of tasks you have. I have 3 icons above so i divided by 3.