FN-3 - is a simple function generator. It is driven by incoming phase (signal from 0V to 10V) and supports 3 modes - sine, triangle and square wave.
Sine mode is a bit different from calling sin function over input directly. When input is at 0V it gives result of sin(x) with x = -1/2 Pi, in other words, it picks lowest point from sin function. Then, as input value increases to 10V the x value is get increaset to 3/2 Pi. Such function remapping gives the full period of sin() function which starts and ends at zero and contains a single hump.
Such remapping is done to make PWM parameter more usable in sine mode. In sine mode it sets ratio between first and second half-periods of phase. Second reasond for such remapping is better usability with bipolar signals and usual envelopes.
Triangle mode is similar to sine mode, but with straight lines on rising and falling half-periods. What makes it special is the PWM behavior, which allow to morph the triangle waveform to sawtooth by stretching on of its half-periods.
There is no any special things about the square mode, it is pretty straightforward. One thing to note is that PWM in this mode allow to turn output to constant low-value or constant high-value on its minimum and maximum.
At its initial idea this module was created to allow creating long envelopes and LFO's from phase information. For example, you can pick a Clock module phase output, stretch it with Divider to, let's say, 4 bars and convert with FN-3 to long envelope or LFO of desirible form which is perfectly synced to main clock.
Such way is not the only way you can use FN-3, you can transform another LFO signals with it, another envelopes and even audio-rate signals.