A tripod is a three legged stand which a camera sits on so it can be very still and stay in the same place while pictures are taken.
Yes
You could say trio as a group of 3.
The Ghost Trio was so named by Czerny because the second movement reminded him of Hamlet's Ghost in Shakespeare. In fact, Beethoven's notes reveal that he was working on an idea for an opera based on...
a trio is a set of three people in a group.