To add on to what Niall said, the 8-8-8s are still allowed as a secondary signal head on smaller roads or for bus/transit signals, but the 12-8-8 is the most commonly used signal nowadays though.
The 12-12-12 is usually only seen when the green section is a arrow, on high speed roads and on left turn signals.
8 and 12 are vehicle signals, 9, 12, and 16 are pedestrian signals.
The 12-12-12 is usually only seen when the green section is a arrow, on high speed roads and on left turn signals.