Why do some loudspeakers have three or more drive units?
There is a reason why loudspeakers have multiple drive units, and. it's not just the way loudspeakers are and that is that.
A loudspeaker that is intended to cover the full frequency range of the human ear has to have at least two drive units.
A large drive unit is necessary at low frequencies simply to shift the large volume of air that is necessary. (At low frequencies, the diaphragm vibrates more slowly than it would at high frequencies, giving it fewer opportunities to push energy into the room).
But a large diagphragm cannot handle high frequencies. For one thing, at high frequencies it starts to bend. The outside of the cone can't keep up with the centre. This causes distortion, known as 'break up'.
Also, a large sound source (larger than the wavelength it is emitting) tends to focus the sound, so people who are not directly in front do not get the full benefit of the high frequencies.
For these reasons, a separate small tweeter has to be used for high frequencies.
But there is a problem if you want a really loud loudspeaker. To achieve a higher level, the bass drive unit (woofer) has to be bigger. If it is bigger, then break up starts to occur at a lower frequency - too low to transfer directly over to a tweeter. Hence a separate mid-range unit is required.
So, the reason for having a big woofer is to get more level, not lower frequencies. And when you have a big woofer, you need a mid range unit as well. Any more drive units beyond three (other than multiple woofers to gain diaphragm area) is generally just to impress potential buyers who don't know any better. Splitting the frequency range into more than three bands causes more problems than it solves.