1 There's "forearm" for the lower arm. If you want a single word for the upper arm, though, you probably need to go to Latin and use "brachium."

2 How about "aftarm"?

3 They're "guns", plural (as in, "welcome to the gun show" /flex).  I've never heard of one upper arm referred to as a "gun", singular, though. 

I suppose this lack of terminology comes from sports reporting?  Most of the time when you hear about a part of a limb, it's in reference to some athlete who injured one. And for whatever reason, upper-arm injuries always get divided into "biceps" and "triceps" injuries.

And wikipedia says the anatomical term "arm" refers to the part from the shoulder to the elbow (I did not know that), so the technically-correct terms really are just "arm" and "forearm".  Silly "common usage" getting in the way. The engineer in me does like the obvious choice of "aftarm", though...