The crux of this system is that the more excitation light (energy) you introduce to the system, the more reflections appear at optical elements, resulting in increased "noise" and in "bright" instead of "deep dark" background. This makes the differentiation between fluorescence positive and negative cells quite complicated. The best solution for preventing such "noise" is to avoid excitation light coming into contact with optical elements of the observation beam path.
The triple beam principle does exactly that; it consists of two parallel beam paths for stereo observation and a separate third beam path exclusively for fluorescence illumination light. The benefit of this principle is that the observation beam paths are free from any direct excitation light. Compared with systems using the same beam paths for observation and illumination, e.g. systems using a fluorescence filter cube with a dichroic filter to separate excitation and emission light, the triple beam principle has a significantly better signal-to-noise ratio.
Moreover, the zoom of all three beam paths is synchronized. The illumination light is only bundled on the field of view of the stereo microscope. By increasing the magnification, the excitation is bundled even more and the amount of excitation light (energy) per area increases.
Another advantage is that the three beam paths are designed to be quite close to each other. The three needed fluorescence filters, one for excitation and two for emission, are mounted in a single small, easily exchangeable slider. Because of its small size, up to four filters can be used in a triple-beam principle stereo microscope setup.
In sum the triple beam principle is a very effective system for illuminating specimens with excitation light and observing the resulting fluorescence with a perfect signal-to-noise ratio. The result is a bright fluorescence signal surrounded by a deep dark background.