Some actions cannot actually be performed for reasons of safety or reality, but must be considered as taking some time to complete. Such actions will have a specific count, usually three, and a definite phrase to indicate what is happening.
For example, to simulate a ghost coming through the wall, the NPC playing the ghost could say “I come through the wall one, I come through the wall two, I come through the wall three.”
During the time in which the player is saying this, he or she is completely vulnerable to attack and cannot use any game skills except Bane, Cloak, Dodge, Evade, Phase, and Resist. These skills cause the counted action to be interrupted. The skill is used up, and the count is aborted and must be started again.
Spell defenses such as Spell Shield or Weapon Shield do not interrupt a counted action.
Example one: Cerik the vampire is turning gaseous. During the three count, he is hit with a Cure Wounds spell. He takes the damage from the spell but it is not enough to destroy him, so he can continue his three count.
Example two: Cerik gets hit with a Destroy Undead spell. This would destroy him, so he uses his Phase skill. The use of that skill has interrupted his three count, so the Gaseous Form skill has been used up, and unless he has that skill twice, he cannot attempt it again.
The three count is used primarily when the NPCs are right there next to the players who can then try to affect the monsters. It should only be used when it has to be made clear that you, the NPC, are phasing in or otherwise taking some sort of action that the player would not be able to observe otherwise.
Monsters coming out from the woods or from behind buildings should not announce their presence with a three count. Why let the player know you are sneaking up on them? As long as you are far enough away that it will take at least three seconds to reach the player, you should not announce your presence. And if you are closer than that, you should be moving farther back.
In other words, NPCs should not, in the presence of players, suddenly take off their white headbands and just appear.
This rule was never meant to mean that you, the player, should count off every action you do.
You do not have to count off drinking potions, repairing your armor, or doing anything else that requires game time to perform.
Basically, counting actions is out-of-game and discouraged unless (a) you are performing a monster ability that has to be counted, such as “rifting in”; or (b) you are in-game performing an action that you are not really performing out-of-game and you are aware that you may be attacked while performing this action.
To better explain (b), imagine that you want to climb out a window. If you really are going to climb out the window, no count is necessary because you really are performing the act. If you are in a module and the marshal says to pretend there is a window there and you have to walk outside and pretend to have come through it, then you are doing the action in-game only and probably should count.
Note that if you are NPCing and are instructed by the marshal of the encounter to do a three count, do so.
There may be specific reasons for it. But in general, let’s do all we can to keep all our actions in-game and count free as much as possible.