This study examined the performance of a booster seat in different seating configurations in side-impact hyGe sled tests (crash severity 30 km/h) with two attachment systems: a standard seatbelt and ISOfix (rigid). The objectives of the study were twofold: (i) to identify the relative benefits of ISOfix attachment compared with seatbelt attachment of a near-side booster seat in a 3-abreast seating configuration with adjacent occupants in child restraints (CRS); and (ii) to examine the effects of 3-abreast seating configurations compared with no adjacent passengers on booster seat crash protection characteristics. Overall, the findings confirmed the superior performance of the rigid anchorages in reducing lateral motion of the booster as well as the two adjacent CRS. However, the expected benefits of the rigid attachment in reducing head accelerations were not uniformly observed across the three occupants/seating positions and also appeared to be influenced by seating configuration (3-abreast versus no adjacent occupant). Further research is warranted to explore the applicability of the findings for different CRS types and seating configurations.