problems with random numbers

I am trying to three random generated numbers so I can get three diferent queries to my MySQL database. The problem I have is that sometimes I only get two numbers instead of three, I sense the problem is that one of the randomized number does not exists in the database. Here is the code, maybe someone could help me? srand ((double) microtime() * 1000000); $randval1 = rand(1, $num_rows); $randval2 = rand(1, $num_rows); $randval3 = rand(1, $num_rows); $rand_text_result = query("SELECT * FROM poems WHERE id=$randval1 or id=$randval2 or id=$randval3"); while ($row = mysql_fetch_array($rand_text_result)) { echo $row["id"] . "<br>"; }