Actually, when you say "3 shells in the magazine", do you mean 3 in the actual magazine, or do you mean two in the magazine and one in the chamber? Because if it's the interceptor latch that's broken - it would be the second shell in the magazine that would be affected. If it's the third or any subsequent shell, there might be a binding problem inside the magazine. I have a recent vintage 870 Police Magnum with this problem. Is your 11-87 a recent vintage model?
Having just had an 11-87 repaired for an interceptor latch problem, here are a few things to look at.
The interceptor is mounted on a post with a "C-Clip" on the side of the receiver. The front of this interceptor contacts the shell coming out of the magazine and holds it with about a 1/8" sticking out of the magazine. There is a small "hairpin" spring which rides in a groove in the rear top half of the interceptor and the other end of the spring is connected to the receiver underneath the interceptor. If this spring is broken or missing, you can almost always have a jam when trying to transfer a shell from the magazine to the carrier. It may work once or maybe even twice but it will jam at some point. This is a fairly strong spring so if you can easily push the interceptor up and down then the spring is definitely broken or missing.
Now if the shell is just sitting in the carrier but not rising to where the bolt will push it into the chamber then there are some other problem areas. Like mentioned above a weak magazine spring or a binding magazine follower can cause the problem. It is also possible the the small latch on the bottom of the trigger group that actually releases the bolt to move forward is bent, stuck or broken to where the shell released from the magazine will not trip the latch to allow the bolt to slide forward.
If the bolt is trying to slide forward with one shell on it, but jamming when trying to chamber the shell, then the carrier may be bent or binding and not properly aligning the shell into the chamber. The hinge point for the carrier may also be worn and causing this problem.
One other thing to check is if the shell is not being released from the magazine, there is a small angled on the action bar on the left (as I recall) side. This small angle trips the shell release. If the angle is worn or broken and does not trip the release, a shell may not feed. Using dummy ammo, with the fore end removed manually twist the action bars one way to see if feeding works correctly or quits working. Then twist it the opposite way. When you twist it so there is more engagement if feeding works ant quits when you twist it away, then that is a definite problem. It is possible to have a good micro-weld shop build up the angle piece very slightly and then grind it to the proper angle and height but know that this is only a temporary fix because you can count on the action bar breaking eventually because of the heat induced change in the metal. Sometimes this problem will only appear when it is cold out. I figure the metal contracts just a wee bit and will expand into adequate contact when it is warm. At least that is what happened to my 11-87 one winter.
From your description, my first guess is a broken interceptor latch spring.
The problem is likely as others have described but just to ask the dumb question - does the malfunction happen with any ammo (new or reloaded) or is it just happening with reloaded ammo?
The reason for this dumb question is that the rims of some shells do increse in diameter a bit after a few trips through a ring type sizer and sometimes to the point they won't feed through tube magazines.
My Rem 11-87 had a problem jamming also
1. Check the magazine tube,I pull out the spring and spring follower, my magazine tube had a lot of unburnt power in it and I changed the magazine follower with the one Shotgun Sports sells (Part number PMGFL for 12ga 11-87).
2.I strip the receiver down bare and and clean it real good.
3.I remove the stock you will need a very big screw driver with a wrench on it maybe. I had to go to a gunsmith to get the stock bolt loose I did not have a very big screw driver, I do now.
4. There is a spring in the tube that's attached to the receiver, when I pulled it out it was full of crud. I cleaned the tube with a bore brush. I had order a new spring and check the length of the old spring and it was almost a inch short.
5. Make sure ever thing is clean good, re-assemble with new O-ring on mag tube.
6. I when to the range no problems, feeds perfect
7. I hope this helps, my 11-87 was purchased when they first came out.And has had a lot shells thru it. This was the first time I changed the spring in the stock.