You cannot use the Raspberry Pi 3 as a USB gadget. At the time of writing this is only possible on the Raspberry Pi Zero, Zero W, and classic Pi model A/A+. The Pi B+, Pi 2, and Pi 3 lack the required pin for OTG.
If you have a Pi Zero, this gist has some examples that work with minimal configuration like g_serial, g_ether, or g_multi.
I'm surprised lukas2005's answer got downvoted. iSticktoit.net has good examples of using the more modern ConfigFS approach. The keyboard HID example is a good one to start with. And when you have only one functional USB port, it becomes particularly valuable to configure multiple gadgets at a time, something that is not as readily available with the old g_* kernel modules.