Not sure if the Sienna you were looking at was a 7 or 8 seater. Here's what I know about my 2008 8 seater. I can have any of my three middle row seats in the car, either all three, two outboard captains chairs, or one outboard captains chair and center seat. The center seat in the 2008 is an almost full-size seat that I can comfortably sit on, and can easily fit just about any Britax car seat on (which is pretty wide, so I would guess I could fit any seat on it). With all three center row seats, I can fit pretty much any three across car seat situation to my knowledge.
But the captain's chair and the center seat are not identical, and I cannot install the captains chair pushed over to the middle like I believe you can in an Odyssey. The center seat can be installed either flush (front/back) with the captains chairs, or in a separate set of floor hooks that pushes it about 3-4 inches forward of the two captains chairs. In all positions, each chair moves forward and back and reclines just like a front row seat would.
In the 2008 (I think it's the same 2004-2010, I know it changed in MY2011) you have LATCH (Lower anchors AND top tethers) in all 3 captains chairs and two of the back three spots (none on the drivers outboard 3rd row), so that spot is not good for a forward facing car seat install, but does work for a RF install or a booster or seat belt user. Three across in the third row would be tight, but I believe can be done with the right combo of seats.
HTH!