There are two ways that an electoral college can become hung. As mentioned above, one of them is a three-way split. But, since the electoral college currently has 538 votes, it can be divided evenly in half, leading to a tied electoral college, at 269 votes apiece. Since neither candidate has a majority, this would head to congress to be decided.
There are 56 units in the United States' electoral college system (the 50 states, Washington, D.C., and the five districts of Maine and Nebraska, that use the Congressional District Method), and they range in value from 1 electoral vote (for Maine and Nebraska's single congressional districts) to California's 55 electoral votes. Someone skilled at mathematics could figure out how many combinations of those yield a tie. On a practical level, however, not all electoral college results are plausible. But there are still a few foreseeable cases where a tied electoral college could happen.
I would say that this is a small detail that we can probably put out of our mind, but I have been wrong before.