Where did the 3 inch length rule for pocket knives come from?

Most adults, if asked, think there is a 3” blade length rule for legal carry of a pocket knife, generally considered a folding knife. Perhaps at one time there was such a generic law, but no longer.
There are two categories of knives addressed under state or federal law: those you can own, and those you can carry. For example, in the vast majority of states you can own a Bowie knife, but you can’t carry one concealed. Some states allow open carry of Bowie knives; some do not.
In Georgia, where I live, you can carry any knife, open or concealed, that is not longer than 12”, with a blade not longer than 5”. That includes switchblades or gravity knives. There are a number of places where you can’t carry any knife, such as schools or government buildings.
In Alaska, where my son lives, you can carry any knife, open or concealed, provided you are 21. But if stopped by a police officer you must tell the officer you are carrying such a knife if it is other than a folding knife. You also can’t carry in certain places, and you can’t carry while talking with anyone who has a restraining order against you.
Under Federal law, if you live in the District of Columbia, or are on any federal facility, you may only carry a folding knife with a 2.5” blade. And, you may not carry any knife of any kind in a number of places, like aboard a plane or in a federal courthouse. Additionally, you may not cross state lines with a switchblade or gravity knife, although it may be legal to carry in your state. For exampleI can own and carry a switchblade knife with less than a 5” blade in Georgia, but the instant I take it into Tennessee, regardless of whether it is permitted under Tennessee law, I just violated federal law and may be imprisoned and/or fined accordingly.
Also, in virtually all jurisdictions the character of a pocket knife may suddenly change because of the intent of the possessor. In the majority of states an absolutely legal folding knife transforms into a dangerous weapon the moment you take it from your pocket, open it, and anyone around you feels threatened by you, predicated on the “reasonable person” concept. If a jury determines that a reasonable and prudent person would have felt threatened under the circumstances, then your legal pocket knife became a dangerous weapon, which is a felony virtually everywhere. In addition, you can be charged with aggravated assault, and if you cut the victim, however minutely, with attempted murder, if your demeanor warrants it.
In summary, don’t take any of the information above as legal advice, because it isn’t. And ignorance of the law is no defense. I encourage everyone to research their state law to know the requirements of a pocket knife. If you own a computer or smartphone it’s super easy. Search for Iowa code, or Oregon code, or whatever your state is. When your state code comes up search it for “knife.” Then read.

Smith & Wesson Extreme Ops SWA24S 7.1in S.S. Folding Knife with 3.1in

For folding knives, it was always about designing a blade that was long enough to be useful, but short enough to still fold and fit in a pocket.
After more than 130 years of experience in the market, most leading manufacturers found that the ideal length was around the 3-inch mark.
From that precedent, a number of governments and legal institutions derived the concept that 3″ or less was about all the average person would ever need to carry, and so put so…

imarku Japanese Chef Knife – Pro Kitchen Knife 8 Inch Chef’s Knives