We own a 2003 Honda Odyssey that we use to transport 5 children (ages 0-7). It has two middle seats (which hold two car seats, obviously) and a rear bench that holds 3 car seats.
There is a space between the middle two seats, but the positioning of those seats can be adjusted so they are right next to each other, with the space next one of the doors.
We would like to transport 6 children.
Is there a way to safely and legally strap 3 car seats into that middle area of the van?
I doubt it, unless the 7 year old is tall enough to ride in the front passenger seat. You won’t be able to fit 3 infant or toddler car seats across the middle bench, much less strap them in.
The OP said that there are two seats in the middle, not a bench seat. I think the question was whether it would be possible to strap one of the car seats to the floor next to the middle seats.
Maybe you could buy another seat to fit in that area, if that area is set up for another seatbelt.
There is no way to anchor a third car seat legally (assuming your state requires car seats for children) where there are only two seat belts. The car seats are designed to be used with a seat belt. Don’t try to jerry-rig it.
Or jury-rig it. (Jury-rig, jerry-built, gerrymander - never the twain shall meet! 
)
Well, whatever. 
But if there’s duct tape, rope, coat hangers, or leather belts involved, don’t do it.
But twine and chewing gum are perfectly acceptable.
You forgot bungee cords. 