I just found a really good article, which I will post below. If you have ever taken a class with me, you know my dislike of using third arms. At first, they seem like a wonderful solve - all for a common problem... "ARGH! I don't have enough arms to solder with." Between holding your pieces together and actually applying the solder, sometimes you need a little help. I try my best to never use a third arm, in fact I do not own one myself, but when I am teaching it is a helpful tool for others if it is used properly.
Third arms can get in the way and actually make soldering more difficult if you don't understand how heat travels through your metal. Trying to solder a joint with a huge metal clamp right next to it is impossible or at best difficult as the tweezer of the third arm acts a huge heat sink, stealing all the heat away from the joint where we want it to be. Proper positioning of the tweezer avoids this heating problem, allowing solder to flow where we want it. Read more below and happy soldering!