Every signal has to communicate two things: what to do now, and what to do as you approach the next signal. Considering that under the NORAC (Northeast Operating Rules Advisory Committee) rules in effect on the NEC there are four operating speeds provided for by signal indication (Maximum Authorized Speed, Limited Speed (45 mph passenger, 40 mph freight), Medium Speed (30 mph), and Slow Speed (15 mph), plus Restricted Speed (prepared to stop within 1/2 the distance you can see ahead of you, not exceeding 15 mph), the permutations of proceed at x speed approaching next signal at y speed necessitate multiple signal units, since experience has shown that only red, yellow, and green can be seen and distinguished far enough away to be reliable. Flashing aspects are also used to expand the possibilities, but even two signal heads are not enough to express all the indications which may be needed at a given location. Traditionally, the a color better than red on the top unit indicated maximum authorized speed through an interlocking, the middle unit medium speed, and the bottom unit slow speed (with yellow meaning restricted speed, leading to the use of flashing yellow for Slow-Approach, when logic would dictate steady yellow). Signals with three heads are normally found only where the specific location requires display of an aspect needing three units, such as Slow-Clear (red over red over green). By the way, most of the signals on the NEC are position or color-position light (these only have two units, since having 4 positions instead of 3 colors gives them more scope) -- where are the 3-unit signals you mentioned?
