The KeyMath-3 Diagnostic Assessment (KM-3) is an individually-administered math assessment used in educational placement and diagnostic decisions. It includes 10 subtests making up Basic Concepts, Operations, and Applications indexes and a "Total Test" composite that measures overall math ability. Here, covariances among subtests from the norming sample were analyzed with confirmatory factor analysis (CFA) to investigate the validity of the three-index scoring structure for identifying relative strengths and weaknesses in math skills. Results indicated a single-factor structure, along with a "hand computation" subfactor potentially representing arithmetic calculation, was the most viable CFA model. The factor is largely measured in the same way across grade groups. At the subtest level, two calculation subtests yielded sufficient specificity for interpretation across all grades; three additional subtests had sufficient specificity at particular grade levels. These findings suggest the KM-3 primarily measures overall math ability and the utility of the three index scores is limited.