Basic motor competencies are defined as motor performance dispositions that evolve from specific requirements in sports and exercise. After validating the MOBAK-1 instrument for first graders, we developed the MOBAK-3 for the assessment of basic motor competencies in third graders. We investigated the factorial and diagnostic validity of this instrument in a sample of 323 children (age: M = 9.2 years, SD = .39; sex: boys, n = 158, girls, n = 165). A confirmatory factor analysis verified the theoretical structure with the two factors Locomotion and Object Control. By means of latent class analysis, we identified five latent classes that were connected with external criteria. We concluded that the MOBAK-3 effectively evaluates the basic motor competencies of 8- to 10-year-old students.