How can I draw the diagram using few decoders for this truth table?
it looks like you nearly have it. 

X is high if the input decodes to the '100 value, 
Y is high if it decodes to '010, '011, '101 or '110 and 
Z is high if it decodes to '001 '011 '101 or '111.

There is one error in your drawing: X, Y and Z are already your outputs. You don't need to "or" them together at the end.



Then take a look at the drawing and see if you can understand how the ABC decoders are redundant and how you can reuse the outputs from just one decoder to compute X, Y and Z so you only need one decoder.