I am only commenting because I can give a little input. I have 3 in my back, but I have a Crewmax. I have a front facing behind drivers seat, rear facing behind passenger seat, and a base/carrier in the middle. There is plenty of space, but it is actually easier for me to put carrier into base via the front, in-between driver's and passenger seat(if that makes sense) instead of lifting it over other cars seats.
On to the point I wanted to make….
There is only two sets of LATCH connections. Each for the outboard seats. For the middle seat, you need to use the shoulder strap to buckle in the seat. I guess you could try to squeeze the LATCH connectors onto the outboard seat ones, but from what I have read, it is not recommended to do that. Not that it's a huge pain to use the shoulder strap, just that it is that much easier using LATCH.
Without looking up the specs, i'm assuming width wise the crewmax and DC are the same? So fitting 3 across is not an issue, and I cannot comment for front to back room in a DC, but I have plenty of room behind front seats to back of rear facing car seats in my crewmax.
Hopefully this helps? Before I bought my crewmax, I researched fitting 3 car seats in a truck, and I came across some forum comments of people with double cabs who said they had sufficient room with 3 seats.