line = "aaa bbb ccc ddd sss qqq eee"

"aaa bbb ccc\nddd sss qqq\neee"

NumberSpaces = 3
counterSpaces = 0
outline = ""
line = "aaa bbb ccc ddd sss qqq eee"
for symbol in line:
    if symbol == " ":
        counterSpaces += 1
        if counterSpaces == NumberSpaces:
            outline = "".join((outline, "\n"))
            counterSpaces = 0
        else:
           outline = "".join((outline, symbol))
    else:
        outline = "".join((outline, symbol))
print outline

''.join([ ''.join([word, '\n']) if enum % 3 == 0 else ''.join([word, ' ']) for enum, word in enumerate(s.split(), 1)])[:-1]

''.join(map(lambda x: x[1]+'\n' if x[0]%3==0 else x[1]+' ', enumerate(s.split(), 1)))[:-1]

import textwrap
line = "aaa bbb ccc ddd sss qqq eee"
indexes = [index for index, value in enumerate(line, 1) if value.isspace() and not index % 3]
newline = textwrap.fill(line, indexes[0])
print(repr(newline))

#Ответ:
'aaa bbb ccc\nddd sss qqq\neee'

line = "aaa bbb ccc ddd sss qqq eee"
newline = textwrap.fill(line, 12)

import textwrap
line = "a bbb cccc dd ssss q eee rr u gggg"
indexes = [index for index, value in enumerate(line) if value.isspace()]
newline = textwrap.fill(line, indexes[2])
print(repr(newline))

#Ответ:
'a bbb cccc\ndd ssss q\neee rr u\ngggg'

indexes[2]

indexes[3]

indexes[1]

indexes[n]