At the moment, the Raspberry Pi 3 box can only boot from the microSD card slot, and not from any of the USB port slots.
So use the USB-SD Creator to format a suitable microSD card for your RPi3, and have it boot from the microSD card.
Actually, the new boot code has been released and you CAN boot a Raspberry Pi 3 from a USB WITHOUT an SD card. Please Google and do the research. This is new code, though, and is not the "official" release. I've done the firmware update on my three Pi 3's and booted right into Jessie on all of them. We're just waiting for LibreElec to get a final release of the firmware, I suppose. For now, I'm looking at other builds, including installing right on top of Jessie Lite.