Newbie prob: How to write a file with 3 threads?

[est]

I need to write a file using 3 threads simutaniously, e.g. Thread 1
write the first byte of test.bin with an "a", second thread write the
second byte "b", third thread write the third byte "c". Anyone could
give a little example on how to do that?

I have my code, but it makes python intepreter crash everytime on my
Vista.