It's unlikely you can fit three car seats comfortably and safely in the backseat of a 2007 Jeep Liberty. While the backseat is relatively spacious for its class, fitting three car seats, especially if they're larger forward-facing seats, will be a tight squeeze and potentially unsafe. The middle seat is often narrower and less accommodating than the outboard seats.
You should absolutely try fitting the car seats in the vehicle *before* needing to use them to ensure a safe and secure installation. Consider the sizes and types of car seats involved (infant carrier, convertible, booster) – this greatly affects the feasibility.
