Hello!
1) As a hunter, you WANT to do your 3rd attack, because it's the most precise. You often need to start your combo when the monster will be stun or won't be able to directly attack you after, it's much more easier with teammates!
2) The trap has a timer, it explode after, OR you can hit the trap and explode it. The most effective way to do so is by using a Twin Blaze, generates Gifoie, and launch your trap which will explode immediately.