Depends where you put it, and how long you had to adjust to using it.
The brain is a ludicrously adaptable thing. Apparently able to fire neurons into networks that’ll deal with everything from existentialism to the existence of Noel Edmonds. A new arm would probably take a bit of getting used to, but that’s what the brain does.
You may have some problem with neuronal to machine interface if it was robotic, and it may also be a little heavy and cold, but assuming we’re talking about organics, it should be fine.
But, I’m not sure where it should go, and whether we’d really get much use out of it. There are definitely situations it would help. Getting keys out of your bag whilst holding things, is the first that springs to mind. Allowing you to wave at people more enthusiastically whilst driving. Maybe if it was put in the right place you could reach that bit on your back that tends to be tricky to scratch.
I’m tempted to point out though, that the human body was ‘designed’ to have two arms, and that seems to work fine for the most part. Balance is an important part of life, and a third arm is likely to be impractical aesthetically and pragmatically. You’ll need all of your clothes redesigned and tailored. Body language will have to be built out of entirely different units. When you need to awkwardly wring your hands because you don’t know what to do with them, you’ll have to either leave one hanging or furiously find a way to make room in the action for a third hand. You won’t be able to peacefully stand with hands folded, without doing some particularly odd bony origami.
Of course, the body isn’t ‘designed’ by anything other than the process of evolution, which doesn’t really have an aim or an intention. The best way to find out the result would be to have half a population with three arms an half with two, and see which one bred and suceeded better through future generations.
You don’t get many creatures in the animal kingdom with an odd set of limbs though. Symmetry and evenness do appear to be selected for, unless it’s just one of those lucky coincidences. The kind of activity that limbs do. Walking, balancing, grasping and cuddling a generally better approached from two directions. The world seems to know this.
If I was going to go for an extra limb, I’d probably go for something a bit more outre. A trunk would be pretty fun. Or maybe a tail. Something I could swish. An extra tool for body language and physical motion. Something good for either trumpeting or dancing.
But human bodies are largely human bodies. It’s going to be a while before we can fundamentally change their shape, and it may be even longer before we’re comfortable doing it.
I’m used to my body how it is right now, and I do quite like it.
Illustration by Henry