My 2 Raspberry Pi 3s won't display on my HDMI monitor (with 2 different HDMI cables) or my HDMIPi screen. Shows as 'no signal'.
Yet if I remove the sd card from the 3 and place it in my Raspberry Pi 2 it displays fine. So it's not the HDMI settings in config.txt!
BUT the Raspberry Pi 3s will work with my Motorola lapdock via HDMI!!