I have never known anyone who owned a 25-3 that had one with proper diameter throats. As others have said, they tended to run large. Usually between .456" - .458". If any ever made it out throated properly, I have never heard of it.
Any S&W model 25-5 (what the 25-3 became when it was put into regular production), with the three letter/four number, serial number, or 25-7 and later guns will have the proper size throats. Some of the later N-prefix serial numbered guns will also have correct throats. To check them follow the tips below.
As the post above states, if you're not sure drop a bullet into the chamber from the rear. I would advise using a jacketed bullet, and not a cast bullet for this test, since a cast bullet will have some type of lube that can stick or hang up enough in the throats to give a false indication of their true diameter. A jacketed bullet works great for this. Use one that is .452" diameter like those from Hornady. Speer bullets tend to run .451".
The best way to check is with minus size pin gauges, which usually run .0002" under nominal size. .4518" for a .452" hole. If they were exact, you'd have to drive them in with something.
Whenever I go to look at a .45 caliber S&W, I take a .452" Hornady with me, usually the 300 grain version, since it is longer and won't tend to tip in the throat giving the impression that it is fitting snugly, when in fact it is tilted and hanging up a little bit. If the bullet drops right through you know you have an oversized issue, but if it catches a little or hangs right up, you should be able to push it on through with a pencil or pen without too much effort.
Rugers tend to run undersized, about .448"- .450", and will generally need to be reamed to reduce pressures, and give their best accuracy. That is about a $50 fix by a good gunsmith, and you only need to send the cylinder for the work.
I hope this helps.
