1.py

multipled = [3 * s for s in series]

2.py

calls = 0
multipled = []
for n in series:
    calls += 1
    multipled.append(3 * n)

print "%d calls" % calls

3.py

class CallCounting(object):

    def __init__(self, func):
        self.count = 0
        self.func = func

    def __call__(self, *args, **kwargs):
        self.count += 1
        return self.func(*args, **kwargs)

timesthree = CallCounting(lambda x: 3 * x)
multipled = [timesthree(n) for n in series]

print "%d calls" % timesthree.count