Make me a loaf of bread before I'm fired!
Charcoal, 35 33 30 bytes
Saved 2 bytes thanks to @ASCII-only
Saved 3 bytes thanks to @Neil by rearranging the order in which the lines where drawn
ＮωＮηＮδ↓η↗→×_ω↑η←\←ω↗δ→/ω↓\↓η↙δ

Try it online!
This is my first try at Charcoal. I heard it was good at ascii-art, so I tried it out.
Explanation (outdated)
ＮωＮηＮδ                  # Take the width, height and depth as input
↗¹                        # Write 1 /
↗δ                        # Write depth times /
¶ω                        # Move the cursor down and write - width times
\                         # Write \
↙↓η                       # Move the cursor one down and one left, and then write | height times
↙δ                        # Write / depth times

Now the back part of the loaf is complete. Now comes the front face.
↑↑η                       # Go up and write | height times
←\                        # Write a \ and move 1 to the left
←ω                        # Write - width times
↓↓η                       # Go down and write | height times
↗Ｐ×_ω                    # Go northeast and write _ width times