Honda RDX Owner's Manual page 55

If you want to install two child seats
in the back seats, place each child
seat in the outer seat, as shown in
the above illustration.
Do not attach two child seat
connectors to a single lower anchor
at a time.
The LATCH system or the child
seats might be damaged if you try to
install three child seats in the rear
seats.
Installing a Child Seat
49