Your code is returning

J
K
D
 
 
W
B
I
R
A
Q
K
F
 
 
Y
E
 
W
V

and the expected solution is

J
K
D
 
W
B
I
R
A
Q
K
F
 
Y
E
 
W
V

You are only replacing the substring with exactly 3 spaces instead of multiple. That means it will not replace any whitespace with more than 3 characters. You need to deal with multiple not 3.

Hmm weird. Post code and I'll check for you.