I got my Raspberry Pi 3 and after I installed Raspbian on it and put the SSH.txt file in the boot folder I tried to connect to it from another computer via Putty. Not knowing the IP, I installed an application on the phone so I can check my network and I can see all the IP's and information about the devices etc...
All devices appear, but the Raspberry Pi 3 is not there.I tried an older version of the Raspbian (Raspian Lite 2017 August I believe) and, same problem, not found on the network. (Yes, I connected the network cable and both of the lights are showing up. The network cable was verified and it works)
In headless installation, you shall save a file named 'ssh' without any extension (I did on mine and it works). ref: https://www.raspberrypi.org/documentation/configuration/wireless/headless.md
To discover the IP of your raspberry, use your browser to connnect to your router (usually is http://192.168.0.1) and ask a list of active connected devices, you should be able to identify the raspberry.