Historically, many monasteries came to be built on Mt. Athos (and are still quite active as a center of Eastern Orthodox activity).

Unfortunately, Mt. Athos monasteries have been attacked at various times in history. In one such attack, one of the defenders of a particular monastery had his arm cut off in battle; (the defense was successful in this case).

Praying to the Virgin (of course surrouded by icons), his arm was miraculously restored. In gratitude, the man attached a metal arm horizontally to a particular icon; it rested on the bottom section of the painted icon. The practice of placing objects such as jewelry, flowers, and other objects on an icon has a long and current history.

So the "Three-handed" icon is actually a much older icon of Byzantine or pre-Byzantine origin upon which an object (the metal arm) was placed.

In some "Three-handed" icons that are full paintings (i.e., no metallic cover), the added arm is actually metal, thus closely following the history above.

On the other hand, when an icon has a metal cover, the portrayal of the added arm may be different. As you will see in the icon below, the hand section of the third arm is part of the painting which lies under the metal cover.