As a simple na-adjective, you should use over in nearly all cases. (But just in case, you may need instead of . means "one is unable to do something", whereas means "something cannot happen". 30 Both mean "That's impossible" in English, but in Japanese they are different.)
is used mainly as part of longer fixed compounds meaning "un-X-able": (incomprehensible), (unrecoverable), (unfixable), etc. In other words, you should think of this as a **suffix** rather than a standalone adjective. You can use in these cases, too, but is shorter and preferred.
is used on its own also in the following _rare_ situations, but intermediate learners may not have to remember these:
* "impotent (sexually)": Outdated. People usually prefer "ED" or today.
* "incapable", "has no ability": Rare and outdated. is almost always preferred today.
* "unsolvable", "has no solution" (math equation): Rare math jargon. is more common in obligatory education.