Can I leave my smartphone turned on in the checked-in luggage?
Your phone must be in airplane mode whether it is in the passenger cabin or cargo hold.  In this case the phone will never establish a 3G connection and won't meet your desired use case.  

Leaving the phone with cell service on will likely also not solve your problem.  Cell phones tend to use a lot of battery trying to connect to a tower when there are none in range.  By the time you land your phone may have already shut down from losing power.  If the bag is lost it may be deep in the bag handling areas of the airport or some other corner with no cell service and certainly no GPS signal.  In all likelihood unless someone mistakenly takes your bag home and decides to charge your phone and turn it back on this plan of yours will not work.