The simple and most logical answer is of course, 3,6 and 9 as all three numbers were not included in what was laid out. But who wants to be simple and logical?
The answer I was looking for is that there are no numbers missing. There were simply numbers listed and you (and I, when I first did this) made an assumption that something was missing. Think about it, while the question asked was what numbers were missing, who said that anything was "missing" in the first place? And if it were those three numbers, who said to stop there? What about the infinite amount of numbers after 10?
The point being, like we may automatically do with other aspects of our lives, we sometimes will make assumptions that things will be a certain way. We "figure things out" long before they've had the chance to play out and take for granted that we know, for example, what somebody else is going to say, what they're thinking and/or how they're going to treat us.
Have you ever had a situation when you need to have something done at work by a certain time? And as that deadline draws closer and closer, you have other things come up that warrants your time and attention. So being the good employee you are, you take care of those other things only to realize afterwards that you now have only 10-minutes to finish your project. What do you do? You start playing out scenarios in your head about how you will need to defend yourself, "knowing" your boss is going to say this, think that and yell at you. But, low and behold, when you tell him/her that you don't have your project completed, the response is something totally different and not really bad at all. No questions, no snide remarks, no threatening of your job - nothing at all like what you were expecting.
What did you accomplish by all the worrying and "knowing" aside from an increased heart and blood pressure rate? We can drive ourselves crazy with all the thoughts and scenarios in a given situation that may happen. But try implementing a process that doesn't put you through all of this. You may just find yourself living a more peaceful and happy life.