Proof of conditional entropy $H( X , Y | Z ) = H( X | Z ) + H( Y | X , Z )$ In information theory, the joint entropy $H(X,Y)$ of a pair of discrete random variables $(X,Y)$ is defined as:
$$
H(X,Y) = -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}p(x,y)log_2p(x,y)\tag{1}\label{eq1}
$$
And the conditional entropy $H(Y \mid X)$ of the same $(X,Y)$ as:
$$
H(Y \mid X) = -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}p(x,y)log_2p(y\mid x)\tag{2}\label{eq2}
$$
With the chain rule of probability, the theorem:
$$
H(X,Y) = H(X)+H(Y \mid X)\tag{3}\label{eq3}
$$
is another interpretation of the joint entropy for $(X,Y)$.
A corollary of this theorem is:
$$
H(X,Y \mid Z) = H(X \mid Z)+H(Y \mid X,Z)
$$
Proof:
\begin{align}
H(X,Y \mid Z) &= -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}\sum_{z\in \mathcal Z}p(x,y,z)log_2p(x,y \mid z) \\
  &= -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}\sum_{z\in \mathcal Z}p(x,y,z)log_2p(y \mid z,x)p(x\mid z) \\
  &= -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}\sum_{z\in \mathcal Z}p(x,y, z)log_2p(x\mid z) -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}\sum_{z\in \mathcal Z}p(x,y,z)log_2p(y \mid z,x)\\
&= -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}\sum_{z\in \mathcal Z}p(y\mid x,z)p(x,z)log_2p(x\mid z) -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}\sum_{z\in \mathcal Z}p(x,y,z)log_2p(y \mid z,x)\\
&= -\sum_{x\in \mathcal X}\sum_{z\in \mathcal Z}p(x,z)log_2p(x\mid z) -\sum_{x\in \mathcal X}\sum_{y\in \mathcal Y}\sum_{z\in \mathcal Z}p(x,y,z)log_2p(y \mid z,x)\\
&= H(X \mid Z)+H(Y \mid X,Z)
\end{align}
I would like to know if it is right.
 A: Without having looked at the details of your proof, I suggest the following alternative argument. 
Instead of seeing $X$ and $Z$ as two distinct discrete random variables, we pair them up and interpret $A := (X,Z)$ as a single random variable. Clearly, all information that is contained in $X$ and $Z$ is also contained in $A$ and vice versa. Similarly, we pair up $X$ and $Y$ by defining $B : = (X,Y)$. We then get by applying rule $(3)$:
$$ H(X \vert Z) + H(Y \vert X,Z) =$$ 
$$ H(X \vert Z) + H(Y \vert A) =$$ 
$$ H(X,Z) -H(Z) + H(Y,A) - H(A) =$$
$$ H(X,Z) -H(Z) + H(Y,A) - H(X,Z) =$$ 
$$ -H(Z) + H(Y,A)=$$
$$ -H(Z) + H(Y,X,Z)=$$
$$ -H(Z) + H(B,Z)=$$
$$ H(B \vert Z) =$$
$$ H(X,Y \vert Z).$$
A: $H(X,Y) = -\sum\limits_{x\in \mathcal X}\sum\limits_{y\in \mathcal Y}p(x,y)\log_2p(x,y)$
$H(X,Y∣Z)=−\sum\limits_{x\in \mathcal X} \sum\limits_{y\in \mathcal Y}\sum\limits_{y\in \mathcal Z}p(x,y,z)\log_2p(x,y∣z)$
$H(X,Y∣Z)=−\sum\limits_{x\in \mathcal X} \sum\limits_{y\in \mathcal Y}\sum\limits_{y\in \mathcal Z}p(x,y,z)\log_2(\frac{p(x,y,z)}{p(x,z)})(\frac{p(x,z)}{p(z)})$
$=−\sum\limits_{x\in \mathcal X} \sum\limits_{y\in \mathcal Y}\sum\limits_{y\in \mathcal Z}p(x,y,z)\log_2p(y∣z,x)p(x∣z)$
$=−\sum\limits_{x\in \mathcal X} \sum\limits_{y\in \mathcal Y}\sum\limits_{y\in \mathcal Z}p(x,y,z)\log_2p(x∣z)−\sum\limits_{x\in \mathcal X} \sum\limits_{y\in \mathcal Y}\sum\limits_{y\in \mathcal Z}p(x,y,z)\log_2p(y∣x,z)$
$=−\sum\limits_{x\in \mathcal X} \sum\limits_{y\in \mathcal Y}\sum\limits_{y\in \mathcal Z}p(x,z)\log_2p(x∣z)−\sum\limits_{x\in \mathcal X} \sum\limits_{y\in \mathcal Y}\sum\limits_{y\in \mathcal Z}p(x,y,z)\log_2p(y∣x,z)
\\= H(X∣Z)+H(Y∣X,Z)$
