You probably want to use timeit module with similar setup:

run.py

from itertools import combinations


def combinations_3(ary):
    l = len(ary)
    for i in xrange(l-2):
        for j in xrange(i+1, l-1):
            for k in xrange(j+1, l):
                yield (ary[i], ary[j], ary[k])


def _test(generator):
    for combination in generator:
        #: To meger time of sequence generation we should ommit IO operations.
        pass


def test_lib(lenght=10000):
    _test(combinations(list(range(1, lenght)), 3))


def test_self_written(lenght=10000):
    _test(combinations_3(list(range(1, lenght))))

test.py

import timeit

#: Print title
print('                library time   self written time')

#: Counter of sequence number.
for i in list(range(10, 100, 10)):
    #: Meger execution time for self written combinations implementation.
    self_written = timeit.Timer(
        #: Tested expression.
        'test_lib({})'.format(i),
        #: Test setup, here we just import tested function.
        'from run import test_self_written as test_lib'
    ).timeit(1000)  #: And here we just set number of testing iterations.

    lib = timeit.Timer(
        'test_lib({})'.format(i),
        'from run import test_lib'
    ).timeit(1000)

    #: Print output in format "<number of elements>: library time, self written time"
    print('{:03} elements: {:10.6f}     {:10.6f}'.format(i, lib, self_written))

And here is my output:

$ python2 test.py 
                library time   self written time
010 elements:   0.007060       0.030495
020 elements:   0.058317       0.239376
030 elements:   0.211884       0.817189
040 elements:   0.522477       1.903142
050 elements:   1.047018       3.803112
060 elements:   1.969069       6.590189
070 elements:   2.960615      10.500072
080 elements:   4.468489      15.176092
090 elements:   6.402755      21.669669
$ python3 test.py 
                library time   self written time
010 elements:   0.009168       0.038298
020 elements:   0.058049       0.285885
030 elements:   0.208938       1.031432
040 elements:   0.523407       2.450311
050 elements:   1.048463       4.796328
060 elements:   1.828277       7.832434
070 elements:   2.970929      13.067557
080 elements:   4.481792      18.496334
090 elements:   6.353836      26.277946
$ pypy test.py 
                library time   self written time
010 elements:   0.011924       0.040272
020 elements:   0.056972       0.046638
030 elements:   0.185858       0.083793
040 elements:   0.456202       0.188640
050 elements:   0.913935       0.338211
060 elements:   1.588666       0.542124
070 elements:   2.537772       0.846942
080 elements:   3.816353       1.232982
090 elements:   5.471375       1.707760

And python versions

$ python2 -V && python3 -V && pypy -V
Python 2.7.6
Python 3.4.3
[PyPy 2.2.1 with GCC 4.8.2]

As you can see, pypy is much faster than cpython implementation. TBC