how to calculate the average (for each three values of y)

 y=[2 3 5   3 4 2   3 4 5   3 2 7  8 6 5  5 4 3  3 3 2 ]

 you should take each three values and find the average for them

   x(1  to 3)= (y(1)+y(2)+y(3))/3 =(2+3+5)/3 =10/3 = 3.33333
   x(4  to 6)= (y(4)+y(5)+y(6))/3 =(3+4+2)/3 =9/3  =3
   x(7  to 9)= (y(7)+y(8)+y(9))/3 =(3+4+5)/3 =12/3  =4
                         .
                         .
                         .
   x(19  to 21)= (y(19)+y(20)+y(21))/3 =(3+3+2)/3 =8/3  =2.66

so your answer will be

x= [ 3.3333 3.3333 3.3333 3.0000 3.0000 3.0000 4.0000 4.0000 4.0000 4.0000 4.0000 4.0000 6.3333 6.3333 6.3333 4.0000 4.0000 4.0000 2.6667 2.6667 2.6667 ]

note: you need to transpose x as final answer