I don't see why it wouldn't work. The 6 year old could be in the middle and the seat would use a regular seat belt for mounting. The 3 year old would be on one side and depending on their weight they could use the LATCH points. The infant on the other side in a seat using the LATCH points. The 6 year old would have to do a little climbing to get in and out, but that's about it. They should be familiar enough to belt themselves up properly. The other 2 would need assistance and so should probably be on the sides as I mentioned before.

Don't forget to have the side door airbags deactivated if you have them. Also don't forget to attach the car seat tethers properly (so many people seem to skip these for some reason). Good luck (and congratulations!).