Truth Table for 3-into-8 decoder with N.A. inputs, P.A outputs and enable
3-into-8 decoder with negative active inputs, a positive active enable and positive active outputs. 


What I like to do for assignments is make a sanity check for at least 3 random cases and see if that checks out, do what I think is correct, then once I'm done, check again, with my first sanity check.

According to what you said, then these 3 expressions should be true:

\$\scriptsize EN = 0, A = B = C = 0 =&gt; D_7 = D_6 = D_5 = D_4 = D_3 = D_2 = D_1 = D_0 = 0\$ 
\$\scriptsize EN = 1, A = B = C = 0 =&gt; D_7=1,D_6 = D_5 = D_4 = D_3 = D_2 = D_1 = D_0 = 0\$
\$\scriptsize EN = 1,  A = B = C = 1  =&gt; D_7 = D_6 = D_5 = D_4 = D_3 = D_2 = D_1 = 0, D_0 = 1\$ 

Let's continue with the rest. 

$$\begin{array}{|c|c|}
\hline E &amp; C &amp; B &amp; A &amp;&amp; D_7 &amp; D_6 &amp; D_5 &amp; D_4 &amp; D_3 &amp; D_2 &amp; D_1 &amp; D_0 \\\hline
       \textbf{0} &amp; \textbf{X} &amp; \textbf{X} &amp; \textbf{X} &amp;&amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0} \\\hline
       \textbf{1} &amp; \textbf{0} &amp; \textbf{0} &amp; \textbf{0} &amp;&amp; \textbf{1}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}  \\\hline
       1 &amp; 0 &amp; 0 &amp; 1 &amp;&amp; 0   &amp; 1   &amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 0  \\\hline
       1 &amp; 0 &amp; 1 &amp; 0 &amp;&amp; 0   &amp; 0   &amp; 1   &amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 0  \\\hline
       1 &amp; 0 &amp; 1 &amp; 1 &amp;&amp; 0   &amp; 0   &amp; 0   &amp; 1   &amp; 0   &amp; 0   &amp; 0   &amp; 0  \\\hline
       1 &amp; 1 &amp; 0 &amp; 0 &amp;&amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 1   &amp; 0   &amp; 0   &amp; 0  \\\hline
       1 &amp; 1 &amp; 0 &amp; 1 &amp;&amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 1   &amp; 0   &amp; 0  \\\hline
       1 &amp; 1 &amp; 1 &amp; 0 &amp;&amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 0   &amp; 1   &amp; 0  \\\hline
       \textbf{1} &amp; \textbf{1} &amp; \textbf{1} &amp; \textbf{1} &amp;&amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{0}   &amp; \textbf{1}  \\\hline
\end{array}$$

I've bolded my sanity checks, which checks out. Inverting inputs is evil. 
And you got your diagonal wrong in your image. 

Here's a sanity check for you that probably got you overthinking things.
\$B = C = A = 0 =&gt; inverted =&gt; 111_2 = 7\$