Enter, LaCrosse - great Party place to be. Have fond memories of visiting a girlfriend who was attending LaCrosse during some big drunken party called Octoberfest. One impressionable weekend to say the least. Now, 24 years later, I think of Dave's Guitar - the nicest dealer for high end stuff in several states.
Back on topic of legal or not. Yes, the easiest way to go is talk to the local DA - that is who will represent the prosecution when you go before the judge on a WEAPONS VIOLATION. Really in the state of Wisconsin an auto is a misdameanor. Please don't take my word for any of these statements - check out your own facts.
The real reason for not asking a police officer is that there is a ton of gray area in enforcing knife laws. You come across as a clean cut fellow who comes into contact with an experienced officer in a rural area and he is more likely to let a 3 1/2" drop point knife go by with no issues. Now, downtown Madison, with a green as they come officer and the LEO asked to see your "Super Death to All Scum of the Earth" Tactical Model from a company that is called "Kill 'em ALL" will most likely be dealt with much differently. Even officers from the same area might react differently. When you are in this situation, now isn't the time to be a smart mouth.
The DA goes to court and has to get a conviction in your case and will set up what is what and that will override some of the gray bull. Guess what though? If you ask and then do it against advice of the conservative DA - you gonna be in the $hit then my friend. Remember that not knowing it was illegal is no defense either, but if you knew and still choose the wrong path - burn him at the stake is what I think your DA would be saying. Is 3" going to be that much less effective than a 3 1/2" blade? Not so much that the risk is worth the small difference that you might see in that extra 1/2". Rambling thoughts of mine - you make your own mind up. Clay