First hit for "three functions of money" was at cliffsnotes.com, why not?
Medium of exchange. Without money, all transactions would have to be conducted by barter ... in a barter system, exchange can take place only if there is a double coincidence of wants between two transacting parties.
Store of value. money must hold its value over time ... Money may not even be the best store of value because it depreciates with inflation. However, money is more liquid than most other stores of value because as a medium of exchange, it is readily accepted everywhere. Furthermore, money is an easily transported store of value that is available in a number of convenient denominations.
Unit of account. Knowing the value or price of a good, in terms of money, enables both the supplier and the purchaser of the good to make decisions about how much of the good to supply and how much of the good to purchase.