Conducting formative assessments can help students grasp the fundamentals of the subject matter and foster a deeper interest in mathematics. To better understand student difficulties in solving word problems related to SPLTV, researchers have conducted studies to analyze the errors made. This research aims to discover and analyze students' mistakes in solving story problems on the material of three-variable linear equation systems. This type of research is descriptive research with a qualitative approach. The subjects of this research were six students from 31 students of class X TKJT 1 SMK Muhamadiyah 2 Pekanbaru, each consisting of 2 students from the upper, middle and lower groups. In this research, data collection techniques were based on observation, instrument test questions, and interviews, which were used to determine students' errors in solving three-variable linear equation systems. In this research, it can be concluded that students' mistakes are a lack of mastery of concepts in three-variable linear equation systems material and a lack of deeper student mathematics learning so that students make any mistakes in solving problems.