Check out random_seed(var val). A random number generator tries to cover all digits for instance between 0 and 65535 in a seemingly random way which however is always the same when you only start with the same value. Thus you have to pick another random seed to get a different sequence with the function mentioned above.
I assume that random_seed(0) grabs some additional information such as for example the clock on your computer which then leads to a calculated number "x" which is then used as random_seed(x) with x not 0.

EDIT: Btw. random(3) only returns values between 0 and lower (!) than 3, it will never return 3.