The Samsung Galaxy S3 is now set to receive many Galaxy Note 2 features. It was a speculation before, but now it has been confirmed as a new Android 4.1.2 ROM which has leaked for the international version of the Galaxy S3 sports many of these new features. the most major of the features that it seems to be getting is the multi window feature (which wasn't available for the Note 2 when it had originally launched, but was added later via an OTA upgrade).
Some other features which you will receive when you upgrade to this new Android version are Smart Rotation (which uses the front camera of the phone to change the orientation of the screen. This feature is usually off.), Paper Artist, a brand new Gallery drawer and the Note 2's keyboard, a modified version of SwiftKey.
The upgrade, as said before is only for the Galaxy S3 international model (GT-i9300) and to do the upgrade you need to root your S3. Be careful if you indeed are upgrading, as there is always a chance of bricking your phone.
Even the older Galaxy Note might be getting some of these features, according to a rumour, which will be very good for the owners of the older Note who might be waiting for some new features on their phone.
Officially, this Android 4.1.2 update should be available by end December.
Source
Link to the ROM