Answer:
x =

y =

z = 0
Explanation:
You first need to solve for a variable, it won't be as easy as x = 3, you are looking to replace a variable with another equation.
For example, the second equation, 4y-z=3, I can either solve for y or z. It won't be a numerical value but it will get you a step closer.
4y-z=3
4y=z+3
y =

Then you are able to use substitution to negate one of the variables and solve for the other two variables. I won't type that but I think you get the idea.