If you have three days, you have to decide on what to see, Toronto, or Niagara Falls. You can hardly do both.
Your dashboard time (sitting in the car and seeing nothing but your own dashboard while on the highway) is ~ 9 h from Chicago to Niagara Falls, ON. That is, of course an impossible number, you have to add in a few stops on the way. Count on one day going and one day to return.
Here is a complete map for you to follow.
https://maps.google.ca/maps?ie=UTF-8&gl=ca&daddr=niagara+falls,+on&saddr=chicago&panel=1&f=d&fb=1&dirflg=d&geocode=Ke3L9NA8LA6IMQDAwAmtpuCv%3BKblNguxeRNOJMYiivVZhxdJG&ei=aFN0UO_xFvGEygHQ2YDIDQ&ved=0CCIQ9w8wAA
The travelling time is about the same if you go to Toronto, but the you cannot see Niagara Falls. To drive from Toronto to Niagara Falls, one way, will take 1,5 - 2,5 h, depending on the time of the day.
If you want to fly quickly and cheaply to the Toronto Downtown Airport (Billy Bishop airport) and perhaps rent a car from there you can fly Porter Airways. That airport is right downtown, serviced by a streetcar stop 200 yards from the front entrance, making it cheap ($ 3.00) to go anywhere in the Greater Toronto area. It is also closer to Niagara Falls by a few min since your literally seconds away from the highway.
https://www.flyporter.com/Flight/Tickets?culture=en-CA
You can save on driving and go for less money by taking a bus from Toronto to Niagara Falls, there are tour buses or regular bus services, galore. To use the GO-service here, take a five min streetcar ride to Union Station.
http://www.gotransit.com/publicroot/en/schedules/pubsched.aspx?table=12&direction=1&day=1&page=1&new=
Or catch a tour bus with some attractions pre-paid. There are many to chose from, here is one:
http://www.niagara-tours.ca/?gclid=CJaT57O09LICFQpgMgodeDEAOA
There are also several train connections from Toronto to Niagara Falls. Again, just take the streetcar to the Union station.
https://reservia.viarail.ca/search/tripresult.aspx?t=D&from=bc
Enjoy your mini-vacation.
.