The theory of constructivism emphasizes the positive and active role of students in the learning process to build knowledge for themselves. Teaching according to the constructivist theory can create more favorable opportunities for applying new teaching methods in practical mathematics education in Vietnamese secondary schools, aiming to maximize the capacity to improve the quality of instruction. For the subject of Mathematics in grade 3, Numbers and calculations are one of the core topics. Therefore, applying constructivist theory in teaching helps students take ownership of their knowledge, seek and adjust their own understanding, ensuring that every day they increase their knowledge and skills.