A requirement is typically a general statement, whereas a use case is typically a specific statement implied or derived from the requirement. A requirement may map to multiple use cases. A scenario might be a set of background assumptions that put a use case in context, or it might be grouping of use cases.
Here is a contrived example. The requirement is to provide an API that converts a CSV file into a tab-separated file. A scenario might be, "John is a Java developer who deals with potentially large CSV files that are sometimes empty." A use case might be "If the input file is empty, the output file will also be empty" or "The API does not run out of memory while processing a 500Gb file".
Finally, SQA gets a lot of "What is the different between these terms" kinds of questions. If you hear someone utter some ambiguous phrases, and it's important to understand what they mean (e.g. because a co-worker or your boss used them), ask them what they mean. Among reasonable people, it is always OK to say, "Different people use that phrase to mean different things, so can you be more specific about what you meant?" If you've never heard the phrases before, or you lack confidence, you can do a Google search first, come up with a first approximation of what they mean, and then ask that person to clarify what they meant.