[21]

Where do you attach a third arm?

[Aardwolf]

How about wirelessly controlled?

[dx87]

Probably on your back, and have it lay flat when not in use. That way you could carry things in front of you, and use the third arm to carry additional stuff as a counterbalance.

[pmontra]

And never ever be able to seat on a standard chair or travel in a car? Probably not.

I would think about how to carry on common tasks with common items such as seating, sleeping, wearing clothes, etc. Perhaps 4 arms would be better than 3 given our symmetry but they're going to force the owner to use so many specially built stuff. I guess two are going to be optimal for a while.

Or attach the extra arms to a hexoscheleton and wear it only when needed.

[21]

The problem is integrating it into the existing skeleton. Our arms are not just glued to the body, they integrate with the shoulder.