Substitution isn't the easiest way. Elimination is easier,
But you specified substitution, so here goes:

(1)  
(2)  
(3)  

Pick any equation and solve for any letter:

I'll arbitrarily pick equation 2 and arbitrarily solve for y



(4)  

Substitute for y in (1)

(1)  
     

Multiply through by 4 to clear the fraction:

     
     
(5)  

We notice that (3) contains the same letters as (5)

Solve (3) for one of its letters.

I'll arbitrarily solve (3) for z

(3)  
     
(6)  

Substitute in (5)

(5)   
      

Multiply by 2 to clear the fraction:

      
      
      
      
      
      

Substitute in (6)

(6)  
     
     
     
     

(4)    
     
     
     
        

(x,y,z) = (2,-3,4)

Edwin