Can generator be used more than once?

This is my piece of code with two generators defined:

one_line_gen = (x for x in range(3))

def three_line_gen():
    yield 0
    yield 1
    yield 2

When I execute:

for x in one_line_gen:
    print x

for x in one_line_gen:
    print x

The result is as expected:

0
1
2

However, if I execute:

for x in three_line_gen():
    print x

for x in three_line_gen():
    print x

The result is:

0
1
2
0
1
2

Why? I thought any generator can be used only once.