ProblemYou've prepped your Raspbian SD card. You've plugged in your FTDI / RS232 adapter to your shiny new Raspberry Pi 3, set up Putty for 115200, Parity 8, Flow Control None, Stop. You've wired everything correctly, Ground to Ground, Tx to Rx, Rx to Tx, and you plug-in, only to be greeted by garbage. Lots of garbage. Blocks, Corners, non-English characters. It looks like your modem looked when it blew off a BBS connection in 1992. But it was just dandy with your Raspberry Pi 2! What gives?
What's happeningBear with me since some of this may be inaccurate. I'll be honest, the last time I dealt with troubleshooting a serial terminal connection was easily 18 years ago. This shouldn't have happened, really. In my teen years, I actually coded a FOSSIL COM driver into Bulletin Board System software I had written for my multi-node BBS (get off my lawn!). 18 years and many lines of code later, I'm feeling senile.
So with serial communication, there's always the sensitive issue of timing. Your Pi is trying to send data at 115200bps. It's failing. Somewhere along the mystical way, the CPU clock rate is set in such a way that it doesn't quite deliver at the right speed and the result is the text vomit you're seeing in PuTTY.
The FixThe fix is simple, but has side-effects. If you're simply setting up the serial terminal in order to login and configure WiFi because you're too lazy to walk 10 feet to another room and plug in an Ethernet cable, it's a perfect solution. If you are interested in using serial communication for some other purpose, it's not.
Simply plug the SD card into your PC. Open up the "config.txt" in the root in something that will understand the line endings properly (Notepad++, Atom, Visual Studio Code, Sublime, ... basically anything other than Notepad). Insert the line "core_freq=250". Save and safely eject the card. Pop it back into your Raspberry Pi 3 with everything plugged in and you should see the majestic, properly formatted, vomit-free boot sequence. Login and you're good!
Once you're done and network connected, remove that line from the config.txt (located in /boot/config.txt if you're editing directly on your RPi) and reboot. You'll be back to the original, faster clock speed (and broken serial communication).