Where does a three-over-three block signal appear?

C&NW/UP,

On the real RR location, I use to see them used on a multiple track area's at a RR Crossing.  The Union Switch & Signal made them for the Track directional crossing areas. However with the advances in technology the triple light stacks were eliminated some years ago.  So if your O gauge layout is actual reality based, few if any still would exist because of the advanced computerized electrical technology allowing the directional lights to be operated from the same computer driven electrical relay system.

PCRR/Dave

The two headed single in your photo would most commonly be placed just prior to the turnout into a siding. Thus, if your train was lined into the siding, the top signal head would show a red indication, while a lower signal head would show either a yellow or green  indication.

The number of heads on any given signal is dependent on what aspects that signal will have to display. An intermediate signal that only has to display Clear (green), Approach (yellow) and Stop (red) only needs a single head.

However, as soon as the signal must display more than those three basic aspects, additional heads are required.  For example:

Approach Medium
APPROACH the next signal at MEDIUM speed (30 mph)

Medium Approach
Immediately reduce to MEDIUM speed (30 mph) and APPROACH the next signal prepared to stop.

Medium Clear
Run at MEDIUM speed (30 mph) through switches or crossovers.
When the whole train is by the signal, then CLEAR for track speed.

SLOW CLEAR
Run at SLOW speed until the whole train is by this signal, then CLEAR for track speed.

There are many more signal aspects than this (Approach Limited, Limited Clear, Medium Approach Medium, Restricting, to name a few) but I think you get the idea.