The Graco Affix is 16.5 inches wide. Due to the slim dimensions of the Affix booster seat, you will probably be able to fit it in three-across installations. However, the high-back booster seat spreads out to 18 inches at the widest point. This is made possible with the spread-out snack tray that adds another 2.5 inches to the overall design. So although it's fantastic for long travels, it might not be suitable for smaller vehicles.