These three adverbs imply the doing or accomplishing of something by the narrowest of margins: "The injured man was hardly (scarcely, barely) able to move his lips." What slight difference exists among them is that hardly suggests difficulty, the hardness of something (My lungs hurt so much that I could hardly breathe); scarcely suggests a margin so small as to be almost unbelievable (You would scarcely believe he could be so stupid); and barely stresses the idea of narrowness and thinness (He barely passed the examination). All three words have a negative quality and should not be used with another negative; do not say "hardly never," "scarcely never," "couldn't hardly," "wouldn't scarcely," and "not barely." See also double negative.
Dictionary of problem words and expressions. Harry Shaw. 1975.