For most people, using the built-in aggregations work. For example, switching aggregation to weighted means and then giving each of the four columns 25% of the grade.
If you really want to enter a formula though, you have to make sure you enter the ID numbers first. When you go into Categories & Items and then select the calculator next to the category total or course total (depending where these items relate to the overall grade), you will then see boxes next to each grade item. Enter an item number for each grade item you are going to use in your formula and select Add id numbers button down at the bottom. (It looks like you have done this already).
When you are putting together the formula, you have to include the double square brackets for each grade item. In addition, use normal paratheses, not curly brackets. So your formula should probably be:
=average([ [E01] ],[ [P01] ],[ [Q02] ])
UPDATE: I had to include spaces in formula because it looks like Moodle converts those double square brackets to create links. So your only problem may have been you were using curly brackets instead of paratheses. You could copy the formula I have above and remove the spaces between square brackets or see image below.
Happy 
oodling!