I do not know about your truck though on later autos the steering arm and steering spindle are all cast as one piece so there is no way to remove the steering arm.
Now the photo calls them steering knuckle and steering knuckle support back then. Same thing.
Now if on your year truck if the steering arm is bolted to the steering spindle you just unbolt it.
Though there should be no need to remove the steering arm for there is nothing to wear out. It is the tie rod end that goes into the hole on the end of the steering arm that needs to be replaced if it is worn out.
My problem is you referred to a "third arm" and I did not see that in those photos. So what are you talking about?
And why are you taking things apart?
