That almost doesn't make sense, heh. If I'm reading that right, it sounds like when you put the third round in, the spring somehow compresses itself all the way to the bottom of its stroke.
Do you mean when you put more than a few in, the spring stops providing upward pressure on the rounds? Two likely culprits for that would be dirty mags or ammo that is too long. Basically, the follower or bullets are getting stuck inside the mag when cartridges are loaded, which is preventing the follower from pushing back up on the cartridges when you release finger pressure from the newly-fed round. Have you tried disassembling and cleaning the mags? What ammo are you using?