This model can only have 2 child safety seats fitted, one on either side - It is not approved to fit a seat to the center location. Only use the Drivers side and Passenger side mid seat locations. The reason the center may not be approved is that the anchorage was designed for lateral loads (side) and may damage the anchorage point in an impact.
For the upper tethers, ensure you use the loop guides provided near the headrest of the passenger and drivers seats.
You may find it easier to remove the headrest, and restore when the installation is completed.