The answer is Congruent Complements Theorem.
The congruent complements theorem states that if two angles are both complementary to a third angle, then those angles are congruent. In the picture, we see that <1 is complementary to <2. But, we also see that angle 3 is complementary to <2. Therefore, because of the congruent complements theorem, <1 is congruent to angle 3.
