in common usage (USA) seminar is very similar to lectures, that there is one lecturer providing info and the audience is more passive. They just listen.

A forum is like a discussion panel, there may be a group of main speakers sitting around a table discussing issues mutually. Open discussions with the audience is very common too.

Workshop, sometimes is a overly-hyped word. But it could mean a seminar, with the typical lecturer and audience. In addition, there will be planned exercises and activities that the audience could do during the workshop to get them more involved.