<< Most states laws on knives are that three inches is the maximum size you can carry around outside your home or residence. >>
That is incorrect.
Knives intended as weapons (double edged daggers) are prohibited. California, which is one of the states with stricter knife carry laws, prohibits automatic knives with _blades_ exceeding 2 (or maybe it was 2.5) inches, and prohibits the _concealed_ carry of fixed blades. There are no carry limitations placed on folding knives (unless they have assisted opening mechanisms or are "butterfly" knives/balisongs). If you want you may carry a concealed folding knife with a blade of 8 inches (and they exist). Certain places to prohibit them though (court in particular).
Intent also defines the legalities of carrying a knife. If you told a police officer, the little Swiss Army pocket knife you carried was for defensive (or offensive) purposes, it would be an illegal carry.
