Aim of the study: to assess maxillary sinus volume changes (MSV) in adolescent patients with class III malocclusion after application of three types of rapid maxillary expanders (RME). Material and Methods: This was a prospective cohort study that recruited adolescent (13-17 years old) class III malocclusion patients with maxillary deficiency. Patients were randomly assigned to three study groups according to the type of RME: Conventional hyrax (CH), hybrid hyrax (HH), and maxillary skeletal expanders (MSE). Cone-beam computed tomography (CBCT) was used to measure MSV prior to and 6 months after application of RME. Measurements were compared between right and left sides and within study groups. P-values of less than 0.05 were considered significant. Results: A total of 51 patients (30 girls and 21 boys) were included. Mean ages were 14.89±1.6, 15.31±1.73, and 15.59±1.9 Y in the CH, HH and MSE groups respectively. Measurements in all groups were significantly increased. In CH and MSE groups, there was no significant difference between the right and left side before treatment (P =0.386).  But in HH, both before and after treatment, the right side values were considerably greater than the left side value (P≤0.05).