I've used the stock Kadee magnets for 3 rail with a modification. I got the idea from a visit to Cape Cod Northerns layout. Rather than dig up ballasted Gargraves track. I cut the metal plate into strips slightly wider than the magnet. I only use one magnet on each side of the rail. Glue the magnet to the metal then glue that to the tops of the ties. I think I used 3/16 basswood strips which will evenly place the magnets between the center and outside rails. The magnets end up being about rail height. I would say just about every car I own uncouple with this setup. As far as spotting cars using the delayed feature. Probably about 80%. The ones that don't seem to work well and are needed for a particular industry are just swapped out with cars that generally stay in a consist.
Bob, with your setup with the middle rail removed. I think this would work and could be disguised either with ballast or a walkway across the rails. I gave some thought to practicing on some scrap track. Using the stock Kadee magnet with the cut strip and then placing those small magnets used for activating reed switches to the under side of the metal strip so that they would nestle down between the ties. Might give the magnet a little more force. I'm not sure of the reasoning behind the Kadee setup with the magnets stacked on top of one another. Not sure if gives more magnetic power or just the fact that 2 are needed because of the higher than scale rails. The metal under the stock magnet seems to be the key.