itertools.islice :

islice(可迭代，开始，停止[，步骤])->islice对象
返回一个迭代器，该迭代器的next()方法从可迭代的.[...]

islice(iterable, start, stop[, step]) --> islice object
Return an iterator whose next() method returns selected values from an iterable. [...]

演示:

data.txt :

line1
line2
line3
line4
line5
line6
line7
line8
line9
line10
line11
line12
line13

代码:

from itertools import islice

with open('data.txt') as f:
    for line in islice(f, 3, None, 3):
        print line,  # Python3: print(line, end='')

产生:

line4
line7
line10
line13