3 inch WILL FIT in 2 3/4 chamber. That it why it is dangerous. If you fire a 3 inch in a 2 3/4 chamber, the shell will open into the forcing cone, overpressure will result. If you are very lucky, you might even still be able to count to 10 on your fingers, and see with both eyes, but if those are things you enjoy, you'd be wise to never do it. The 3 inch size refers to the length of the fired shell, they are actually 2 3/4 in length when unfired, which is what leads people to make mistakes of this nature if they are not paying attention. Be careful. This is most definitely not a stupid question to ask.