I can give you a general feel from years of attending and reading proceedings of all the different titles you mention...
1) Conferences - these tend to be "show and tell". For the most part, all of the members of some organization get together to tell all of the other members of the organization what they are researching, how they are researching it, and what answers they are getting. The organizations may be very rigorously defined and long-lasting, such as the American Chemical Society, or may be ad hoc groups with a similar field of study gathered together by a conference promoter. In general, topics are self-generated, but with large conferences, presentations tend to be grouped by at least the general area of study.
2) Symposia - these usually cover a specific area of study, usually one that is dealing with either areas of controversy or areas of scientific or political necessity. The object is to gather the experts in the field with a wide variety of opinions, and allow them all to present to an interested body. The interested body may be politicians or activists who need to be aware of the state of the science to form political consensus and mold policy, or they may be other scientists who are developing the body of opinion which will inform scientific paradigms and scientific education. Thus, there are symposia on climate change, cancer treatment, Alzheimer's disease, and so on. I find well-planned symposia to be the most interesting of the scientific gatherings, the results of which are often a lot of people developing new experiments to try to prove one or the other viewpoint wrong.
3) Seminars - these are typically educational events. The purpose of seminars is to provide experts in a topic area to inform a non-expert gathering so they can use the information presented. Typically, seminars are presented on topics with broad expert agreement, rather than areas of scientific controversy, and the purpose is to pass on the information rather than to inform opinions. Seminars are often organized around techniques developed in one field that are either frequently misunderstood, misapplied, or unknown but potentially very helpful in another field. For example, seminars on analytical techniques for organic chemists, or seminars on pharmacokinetics or toxicity to medicinal chemists, or seminars on lab safety to just about anybody.
Just my viewpoint - other people may see other definitions in these words.