Shooting multiple arrows is somewhat similar to throwing knives.
Certainly, it is possible and it isn't completely useless, but it's not something commonly done in a combat situation. Its unreliable and requires many hours of practice to perform with any modicum of skill.
With Knife Throwing, there are many other weapons that are far more effective at range. Instead of wasting your time trying to throw a knife, just pick up a bow or throw a javelin. The knife is just a backup weapon at close range and a tool.
Firing a second arrow will reduce the power of the bow, but if its being done at close range you can afford to sacrifice power against lightly armored targets. The real issue here is that you don't gain much. Let's say you fire two arrows at a single target. If one of them misses, the second one likely will as well. You've handicapped yourself for no benefit. One arrow in the chest is enough to fell most targets, so adding a second one isn't going to gain you much.
You'll be better served to simply draw and fire one arrow using the same technique you've trained and practiced hundreds or thousands of times rather than train a whole new one.
Now, just like with Throwing Knives, if a person was an expert that spent all of their time honing their skill then they would be quite fearsome. A normal person throwing their knife is just a somewhat sharp rock that needs to be dodged, while an expert can clip the wings off a fly at twenty yards. 523
The same could be said of firing multiple arrows. Its certainly possible for a person to train so long and hard they could fire two arrows and strike two different targets(with reduced power), but who has that kind of time to devote ten or so years to constant training?
Just look at <PERSON>. He's firing arrows faster than anyone thought was possible and can even fire them around obstacles. Of course, this is done with special arrow nocks and a weak bow, but he is a 50 year old man reinventing this stuff. Imagine young and extremely strong men with skills being passed down over many generations.
It's mainly an argument over the way that an army or large force would do things and the things that isolated experts can do. In your question you ask specifically about a multi-arrow bow. That would likely be the only way this sort of thing would work, creating special bows/equipment to help reduce the amount of training needed by the rank and file.
The bow might have large V-channels on it for the arrows to fall into. Quivers could have grids in them so the arrows are easy to grab in groups. They might even have special drawing devices that not only grip the string but keep the arrows spaced properly.
Of course, you'd want there to be a valid reason to do this. Why would you want to fire three weaker arrows rather than a single more accurate and powerful one? Is there some fantasy beast with three equally spaced eyes that are its only weakpoint? Do they have a special tri-part magic technique that needs three arrows flying in parallel to activate?
If there is a reason to do something, then people will go above and beyond to figure out a way to make it work. 300