The Prius was redesigned for 2016, and sadly it wasn’t a good change for anyone wanting to put 2 side-by-side, or have 3 of anything in back. The center is now a completely useless spot – it is so narrow that no car seat, booster, or adult can fit in it. We’d suggest getting the Prius Prime which is the same back seat except that the center seat is replaced with a console – so it won’t tempt you to try and fit more in the back than you can as it realistically and truthfully represents itself as having just 2 seats in back.
In the 2016 & newer Prius, the only 2 seats that are usable are 2D & 2P. There is nothing that fits in 2C.
The Prius was refreshed for 2010 – the only notable change was that the head restraints for 2D & 2P now flip up/down and are removable (the same head restraints will continue in the 2016 & newer models as well).
The Prius was redesigned for 2004 and introduced exclusively as a hatchback. There are now head restraints for all 3 seats. There is seat belt overlap between 2C & 2P – but it can be overcome with some narrow combinations of car seats as all the seat belt buckles are flexible.
The Prius for 2001-2003 was only offered as a sedan. See here for more about this Prius.