How does the ambush ability work, that the wood elves use. Could someone please explain? Thank You!

After attackers are declared and the Action window that follows, there is now an ambush step. If you have any facedown cards (developments) in the zone being attacked that have the ambush keyword, you may now pay the cost that follows the ambush keyword to flip the card face up. If it's a unit, that unit will automatically be declared as a defender and it will generally have an effect that happens when it comes into play with the ambush effect.

But what does ambush 3 mean? What does the value mean? And also, how does necromancy work?

Ambush X means that for X resources you can turn that development face up during the ambush step. Junglecat said it pretty clearly.

Necromancy:

You can play units from your discard pile as if they were in your hand - pay their cost and play them into the zone of your choice from your discard pile. Then, if it is still in play at the end of your turn, remove it from play and put it on the bottom of your deck. Again, pretty straightforward from the rulesheet.

I am sorry, but I never got any rules which explained it. I assume the rules are in marched of the damned, but I got the hidden kingdoms expansion. Anyway... Thank You!