We were able to make it work by putting SB, who is almost four, front-facing in his five-point-harnessed Radian65, behind the driver; GI, who is 14 months, rear-facing in his Radian R100, in the middle; and NJ, who is six, in a booster seat behind the passenger. While I would have preferred to put NJ in the middle, it would have been next to impossible for him to get in and out easily and to buckle his seatbelt - really, the same problem he had in the Highlander.
SB's Radian65 was installed using the LATCH connectors, GI's Radian R100 was installed using the seatbelt, as there were no LATCH connectors for the middle seat, and NJ, of course, used the seatbelt with his booster.
It worked out well, and so I am pleased to report that, for those of you trying to get three across in the back of a sedan, it is entirely possible in a 2012 Toyota Camry using Diono Radian seats. I don't think it would have been possible with wider seats, such as the Graco Nautilus. It probably would have worked with an infant seat with a base, as well, although we had no reason to try it.
Also, don't miss my video demonstrating how to properly harness a four-year-old in a five-point harness, the inaugural video of my new YouTube channel. Subscribe now to see future videos, which will typically integrate with a related blog post!