Tipu, the three years rule might not apply to all materials. Many US produced documentaries and materials have a copy right extending tens of years. I would suggest that if the video or movie available to download or watch by the original producer, then it is a safe area to post provided that you link the producer page.
Regarding what section you can post your threads to, it is hard to give you a list of what fits in and what fits not. I would suggest you follow those two points before posting a new thread:
1- Search the website for similar threads and once you find one, just post your post under that existing thread. For instance, you would like to learn about Mehmet Ali Pasha. You enter that in the search option and you might find your answer in an older post, or you might post your contribution to that older thread and your respond will appear in the "active topics" again for other members to read your post and comment.
2- If it is a different topic not discussed before, then take your time looking into all forums and sub-forums to decide where it fits best according to the subject. This step will be easy with time as you learn the categorization of all forms here.