One of the most important and fundamental reasons for learning mathematics is the ability to solve word problems. Exposing students to various problem-solving strategies, such as the heuristic approach, can assist them in becoming expert problem solvers. The primary goal of this study was to determine the effect of a heuristic approach on improving students’ problem-solving performance in Grade 3 Mathematics. In accordance with this, the researcher developed the hypothesis that was no significant difference between the pretest and posttest of students who used modular and online learning modes. It also revealed which of the five problem-solving strategies use the best. This study employed a quasi-experimental design with no control group. The researcher purposively selected two sections of Grade 3 students from Gatid Elementary School in the school year 2020-2021 as the two experimental groups; 14 students for online learning approach and 26 students for modular learning approach. Students who took part in the study completed likert-type surveys, content exams, and were interviewed individually. Participants in the online and modular learning approaches were selected based on Likert survey data and interview responses. A questionnaire checklist with a pretest and posttest was used to collect the necessary data from forty Grade 3 students at Gatid Elementary School in the Division of Laguna. Mean, Standard deviation, Parametric t-test and Cohens D were the statistical tools used. When integrating a heuristic approach in teaching problem solving, it was discovered that the learners did not have equal cognitive skills. This provided a medium effect on the improved performance of the students using modular. Furthermore, the heuristic approach had a small effect on the posttest performance of the two student groups.