Gears & Axles - 3 series internals in a 2 series rear-end

can you put a 3series open diff in a rear end that has a 2 series open diff, the 2 series rear end would be gutted and the entire 3 series rear end would go in, would this work?

Yes, the pinion, ring gear, and diff unit must stay together. Meaning as long as you have a 3 series of all of the above it will work. Or, if you have all 2 series of the above it will work also.