I keep coming back to this fact: The action works perfectly for the first two, but fails to eject the last shot fired and stay open (maximum capacity). The recoil spring seems very strong to me, so I fail to see how that could be the problem. The extractor and ejector both work well for those shots.
I've even tried firing the Express (1 1/8 ounce) loads without the extra ring with no improvement.
The first thing I did was to disassemble and clean the action. I adjusted the carrier, whose screws were loose, causing the carrier to have too much horizontal movement. Now it works very well, without binding or excess play.
Since the carrier doesn't lift on the last shot, (or only shot), perhaps the latch is at fault. I noticed that the action closing button on the right side seems to have play in it, so the latch may be misshaped?