Part of this really depends on where you are learning, how often the classes are, how big the class is, and the curriculum and structure of the class. Given that you haven't mentioned what style of tai chi you are learning, the movements/forms, and what your instructor or class format is like, I can only draw so many conclusions.
For instance, if your class is full of beginners and exercises or forms are conducted together in a communal learning environment, then I wouldn't expect to progress so quickly. Remember that you were once a beginner too. The instructor's time and attention has to be divided among all students. Some people, like very new beginners, may need more explanation to understand what they are doing.
The way you pose your question does in fact make you sound very selfish in wanting more, like you have this unearned sense of entitlement and expectation based on what has taken place so far. To demand more material from your instructor is very disrespectful and also very inconsiderate to others. People I've seen who think they are ready are always not because their understanding is so superficial. You can choose to approach your instructor is fine, but please be aware of your place in the class and you are not running it.
I would not be concerned about "losing out" just because you have not advanced in anything over the last three classes. I am not understanding why you seem so concerned regarding this. If you feel like that what you're doing in class is useless or isn't right for you, then you are always free to choose something else or go somewhere else. Try to open your mind and accept that doing something different and unexpected can be beneficial.
Just because you disagree with the teaching style or format of the class does not mean there is anything "wrong" with it. Regardless of what the curriculum is, sometimes the class will just have to be tailored to people there, what people are doing together as a group, the materials and forms people are learning, and so on. What goes on in a class is really up to that instructor, and that person should reserve the right to modify things as necessary.
I would not expect any tai chi student to really begin to fully grasp what he or she is doing after only three months. Even as an "advanced" student, I am still drilling and practicing fundamentals because they are so important; the are the basis of everything we do. Having a foundation is important, and you can really only get better from there. I wouldn't expect anyone so "young" in tai chi to make any huge advancements for a good while. All I can say is: practice, practice, practice.