Usually, attacking 3 times is a pretty good option for monsters, since they have such inflated attack bonuses. It is really good to model good tactics for your players if you remember to, though. (I usually forget to do this.)

For more intelligent monsters, have them step back out of melee range with their 3rd action, or open with a Grab, Trip, or Demoralize. Those are actions any monster with the skill can take.

Generally if a monster has a unique action, you should use it as soon as applicable (even if it's not technically the best action to take). Monsters have limited screen time, so make sure to showcase the unique things they can do!