1) Life is but a walking shadow. (from Macbeth by William Shakespeare): metaphor
A sentence uses metaphor when it directly asserts that one element is similar or equal to another element (not literally but figurately) without using the words "like" or "as". This line uses metaphor because it directly compares life to a walking shadow.
2)He could feel his heart pounding and then he heard the clack on stone and the leaping, dropping clicks of a small rock falling. (from For Whom the Bell Tolls by Ernest Hemingway): onomatopoeia
Onomatopoeia is a word or phrase that imitates natural sounds. The excerpt uses it because it contains words like "clack" and "clicks" that imitate the sounds they describe. Other examples of onomatopoeia are "smash", "clap", "pop", "bang" "cough" and "moo".
3)Touch each object you want to touch as if tomorrow your tactile sense would fail. (from The Seeing See Little by Helen Keller): alliteration
We can identify alliteration in a text when two or more neighboring words have the same initial consonant sound, such as in the given line: the words "touch", "tomorrow", and "tactile" have the same initial consonant sound of "t."