I use Raspberry Pi 3 with image "RASPBIAN JESSIE" which is 2016-02-26 build. I plug in micro-usb to power on. Then I use USB to TTL adapter to try to login. I've set baud rate 115200, and connect GND, Pi's TXD0 to TTL's RX, Pi's RXD0 to TTL's TX. But I only saw mojibake. I've tried other usual baud rate and comes same results. I also try use this image on Pi 2, and it behaves normally. Does anyone knows how to login with uart on Pi 3?
I found answer here: https://openenergymonitor.org/emon/node/12311
Briefly it's due to Pi 3 default enable bluetooth which supplants uart on GPIO 14/15. So we need to disable bluetooth.