This is totally dependent on the particular drug product you are using. One rule will not work for all products. You are right, Serostim is 3iu per mg.
ItalMonster, there are 100units per 1 ml for u-100 insulin ONLY. You cannot apply this to other products, and it might be dangerous to do so. There is a u-500 insulin for insulin resistant patients which is 500 units per 1ml, for example. So don't use your "100 units per 1ml" rule for anything but u-100 insulin.