Trout, you should be fine.
There is not really a >3" prohibition here (it's pretty much a myth), although as indicated above, there are some limitations based on age, and *intent* of the knife. If you wear it outside your belt, and it isn't some egregious Bowie (you didn't say what type it was), no one will think twice.
Where are you going? Note that if you'll be in the Loop, a number of buildings (notably the Sears Tower) will confiscate it at the security gates. As it happens, a colleague of mine forgot that he had his folder in a pocket, and they "held on to it." He got it promptly returned as he was exiting the Tower.
Now if you take any blade of any length out and start waving it at people, you'll be very sorry.
But weaing one in a belt or popping a folder in a pocket is not illegal. As said, balisongs are all right, too. I carry one in my car... but again, it's what you do with it. If you use it as a weapon in any way, it's illegal.