from itertools import izip_longest

def grouper(n, iterable, fillvalue=None):
    "grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx"
    args = [iter(iterable)] * n
    return izip_longest(fillvalue=fillvalue, *args)

yourlist = ['one', 'two', 'three', 'four', 'five', 'six', 'seven']

for line in grouper(3, yourlist, None):
    print ', '.join(filter(None, line))