How can a graduate student most efficiently become an expert at his research topic?
For most disciplines, advanced textbooks still provide, albeit necessary, foundations. Conferences allows you to hear what is new and is as such a good way to know where fields are going. I am not sure what you mean by seminars but discussing new papers and topics is always a good way to advance knowledge. But, the major point is to read up on published papers and to focus on the topic of your thesis. The point of research is to advance knowledge and this means you will, or at least have a chance to, be an expert on the field you are researching. So in my view none of the activities 1-3, or combinations, make you an expert, only your own sweat over publications and own work will do so.