The first argument is usually the one you need, the other 2 are optional.
The first one: the number you enter there is the value in percent, by which the game will be played faster/ slower.
The second one: is the time in milliseconds for the new value to take effect, means if you use the default speed on mission/ cutscene start, and don't use the second argument, the game will instantly increase/ decrease speed to the new value.
With a value entered, the acceleration will run smoother.
Think about the movie Matrix:
when they switch to bullet time, they start from normal speed, then there is a noticeable slowdown before they show the rest of the scene in the bullet time speed.
Just check the scene on the roof where the Agent is firing on Neo, shortly before Trinity is popping a bullet in the Agents head, that's a perfect example.
The third: the value from which the new value will start from, means: usually you star from default, with the third argument you can start in slow-mo/ fast forward then it will increase/ decrease to the new value.
You might wanna check Vidmaster's Dark Children campaign, it's using "the feature".