In Ardour 2.X, turning on chase-timecode was done by choosing a timecode source or switching back to "Internal timecode". This meant that switching between chasing and not chasing external timecode was more cumbersome than it should have been.
Ardour 3.0 puts the choice of positional synchronization ("timecode") source (MTC, JACK, MIDI Clock etc) in the Session->Properties dialog. This means that the actual timecode button in the transport bar can be used to just turn chasing on and off, without losing the currently chosen timecode source.
The screenshot shows the controls for MIDI Timecode (MTC) as the timecode source. To the right of the dialog, you can see the transport bar's timecode button displaying the selected sync source. The button will flash (red/green by default) periodically if Ardour fails to lock to the chosen timecode source, thus assisting with diagnosis in cases where MTC is being lost or mistransmitted. Once lock has been achieved, the button "lights" permanently to indicate this. If lock is lost, the button will start flashing again. Note that as in Ardour 2.X, Ardour will mute outputs when lock is lost.
Below the timecode button is the Time Master button (used with JACK Transport to make Ardour be in charge of the JACK transport position time information). The operation of this button has not changed in Ardour 3.0.