You might see if this paper helps:

> Ranestad, Kristian, and Bernd Sturmfels. "On the convex hull of a space curve." arXiv:0912.2986 (2009). ([arXiv abstract link](http://arxiv.org/abs/0912.2986))

<hr />
&nbsp; &nbsp; &nbsp;
![CurveHull][1]
<br />
&nbsp; &nbsp; &nbsp;
<sup>The yellow surface is $z - 4x^3 + 3x = 0$.
The green surface has degree $16$.
The pink triangle is planar.
</sup>
<br />
&nbsp; &nbsp; &nbsp;
<sup>
(Image due to  Frank Sottile,
Philip Rostalski.)
</sup>
<hr />
If an approximate hull would suffice, it is "easy" to compute the 3D convex hull
of many points along your curves.
Here is a crude attempt on your two curves $A$:
<hr />
&nbsp; &nbsp; &nbsp;
![CurveHull][2]
<hr />


  [1]: https://i.sstatic.net/zPJpr.png
  [2]: https://i.sstatic.net/jP3SW.jpg