Problem-solving is a key aspect of mathematics instruction. Although it is part of several national curricula, its implementation is still being vastly explored. The present research, which is part of a larger project, aimed to investigate the effect of using problem-oriented instruction in grade 3 of primary school on students' mathematical abilities of solving problems in arithmetic, geometry, and logic. The study used a pedagogical experiment, with a sample of 240 pupils in Grade 3 from four randomly selected Slovenian primary schools. Students were divided into two groups: 100 in the experimental group and 140 in the control group. Two tests of mathematical knowledge, constructed by the authors, were used to assess potential differences between the groups before and after the experiment took place. The results showed that the use of an experimental model of problem-based learning had a statistically significant effect on pupils' mathematics problem-solving achievement in all the domains of mathematics (i.e., arithmetic, geometry, logic & set theory). The study suggests that modern school reforms should focus on problem-based mathematics instruction as a means of achieving meaningful and lasting knowledge in students. Additional research is needed to replicate the results due to the limited number of participants in the present research.