What is the missing reason in step 3?

Answer:- The missing reason in step 3 is that:

Vertical angles are congruent.

This is about angle theorems as used in each of the steps in the question.

Now, in step 2, we see that

∠EDF and ∠ADC are vertical angles. This means that both angles which are opposite to each other are made by 2 intersecting lines which are A F and EC.

Now, in step 3, we see that:

∠EDF ≅ ∠ADC

The symbol ≅ means congruent.

Moreover, according to vertical angles theorem, it states that vertical angles are always congruent.

Thus, the missing reason in step 3 is that:

Vertical angles are congruent.