Tricky Regexp - How to insert a marker every 3rd number in a sequence that begins with a certain delimiter

[gnuist006]

tricky regexp

How to insert a marker every 3rd number in a sequence that begins with a 
certain delimiter, and ends with a certain delimiter and its length is a 
multiple of three?

I want to isolate sequences like this in a text and to work on them only.

Given:-

text
text
BEGIN N N N END
text BEGIN N N N N N N END
some text BEGIN N N N N N N N N N END
text
N N N N N N N
text

The sequences I want to work on start with BEGIN and end with END with exact 
multiple of 3 B's in between with only single space. I want to place a newline 
before every 3 B's. So the above text would transform to

text
text
BEGIN 
N N N 
Z
text BEGIN 
N N N 
N N N 
Z
some text BEGIN 
N N N 
N N N 
N N N 
Z
text
N N N N N N N
text


More accurately, in a "BEGIN N N N N N N N N N END" type sequence, I want to 
insert a \n before every N whose cardinality is 3n where n=0,1,2,... and the 
first N has cardinality 0. I also want to insert a \n righ before such a END.


My efforts:

(replace-regexp "BEGIN \\([0-9]\\) \\([0-9]\\) \\([0-9]\\) END" "BEGIN \n\\1 
\\2 \\3 \nEND")