I find that shooting 3's and longer-ranged shots in NBA LIVE 08 Pc is extremely difficult. Unlike the consoles, there is no bar to indicate the release, but the AI seems to be doin it no sweat Bigger/stronger player =release later?
You can always adjust the sliders to make shooting threes easier. Go to options -> gameplay settings, select "user" in the bar on the top of window with sliders, then find "long range shooting" and set it to bigger value. After that more long shots will go in.
To get to know when to release shots you just need to practice. There is no other way.