The IATA (the organization that regulates the airlines) recommends that all West to East and South to North flights have even numbers and the opposite directions have odd. This is the popular trend but there is no set rule that the airlines have to follow.
Another popular trend is that 4 digit numbers are either international flights (any flight leaving the country), codeshare flights (a flight under one airline’s name that is flown by another airline’s plane and crew), or flights into small airports (such as New York to Buffalo).
While 3 digits are domestic flights. You will see however that there may sometimes be an exception with international flights that fly heavy flown populated routes (such as from New York to Paris or Los Angeles to Hong Kong).