You ll be on the blue line 40 min each way between ORD and Washington/Dearborn then a 2 block walk to/from Millenium Park. Add to this the security line at the airport and you can spend about 30 min. looking at Cloudgate.
You ll be worrying about getting back in time, especially if you have luggage to check again.
Sorry - 3 hours isn t nearly enough time to get from Ohare to the Loop and back again.