How can I use Magic Keyboard, Magic Mouse 2, Magic Trackpad 2 with late 2011 MBP? I have a MBP 17" (late 2011) and would like to use the new Magic input devices (Keyboard, Mouse 2, and Trackpad 2) with it which seem to rely on Bluetooth 4. How can I connect them to my MBP?
You could just use it wired, meaning connect the lightning cable that is usually used to charge the pad to connect it to your MacBook, it works.

Some people tried using it with older Macs and it seems to work with Bluetooth 2.0 as well, but i doubt the battery will last as long as with Bluetooth 4.0.

The chip in the device itself is only capable of Bluetooth 3, but Apple never sold Bluetooth 3, they went from 2.0+EDR directly to 4.

You will however need El Capitan, without that it won't recognize the device.