I'm not sure you can fit 3 across in the rear. You can push the seat out a little more on each side but it would be hard with the latch anchors.
Legroom is not bad for the passenger. Obviously you will want to get the angle adjuster for the Diono's. Also I ended up purchasing this to put against the top of the car seat to prevent the car seat from putting indentations into the headrest (Armacell Tubolit 3/4 in. x 6 ft. Polyethylene Self-Seal Pipe Wrap Insulation-OES07838 - The Home Depot)
That makes sense regarding the LATCH system. I suppose if you move the car seats farther out then they may not align well with the LATCH anchors...