Michigan has the firearm preemption law (MCL 123.1102) that prohibits local municipalities from enacting any firearm ordinances more strict than existing State law. However, there is not any similar preemption law that covers knives so... even though State law mentions a blade length of 3 inches there is nothing preventing your town, township, county, or city from banning knives of all kinds regardless of blade length.
Because of this I would strongly suggest folks check into their local ordinances to discover if that little jack knife they have in their pocket qualifies as an illegal concealed weapon.
Also because there is no knife preemption I would strongly suggest folks who carry knives, including the ubiquitous jack knife, check into the ordinances for all the localities they go to or travel through on a daily basis since each municipality might have different knife ordinances.
And I would doubly strongly suggest folks who carry guns rethink also carrying a knife, even a little jack knife, because if arrested/convicted/confessed to carrying a knife in violation of a local ordinance it might put a person's carry permit in jeopardy.