This was an issue that was fixed some time around April 2014. See balpha's answer (he is an SE dev) for more information.
Now you can make only the first letter bold (
**b**old), using just asterisks. (Fun fact if you just click the edit button on your question you will see the live markdown now displays correctly.)
However the triple asterisks (
***b***old) wont work. you have to use
**_b_**old which will render bold and italics. However if you want an entire word bold and italics you can use the three asterisks.
Another thing that has changed is underscores in a word are completely ignored. Here_is_an_example_
Yet the same thing with asterisks will display like this: Hereisanexample