Distinctions in the meanings of these words have largely broken down, but impractical actually means "theoretical" or "speculative." Impracticable means "not capable of being used," "unmanageable." "The architect's recommendations are impractical, and his blueprints are impracticable." Unpractical is interchangeable with impractical but is considered less formal.
Dictionary of problem words and expressions. Harry Shaw. 1975.