They fit in fine, but when the crimp opens up it does not open up in the chamber but in the cones, restricting their width, leading to the first of your two options (It depends on the size and angle of the cones.)
Recently the Double Gun Journal did some tests on 2-3/4" and 3" being fired in 2-1/2" chambers commonly found in many English guns. No kabooms reported.
It's always recommended to use the right length shell in a gun because while some guns might withstand the added pressures other may not. That said, I don't live in paranoid fear of the prospect of accidentally dropping a 3" shell into a 2-3/4" chamber. I DO live in fear of accidentally dropping a 20 gauge shell in a 12 gauge because that can cause a kaboom if the 20 shell slips into the chamber and a 12 is put in behind it.
I once blasted off a bunch of 3" shells a friend gave me in my 2 3/4" Wingmaster. I wondered why that dang things would only fully eject if you racked the slide with authority. I figured something was wrong, and once I stopped to think about it a moment...say, aren't those shells a little longer than normal?
Ah, to be young and stupid again. The gun did not kaboom. Shows no sign of anything wrong. I'm sure if I tried another stunt like that today, Fate would take notice of the fact that I'm no longer 15 years of age and cause the thing to detonate on the spot.