Onomatopoeia is reproducing sounds from the nature like saying splash for describing water sounds or woof to describe dogs.
Alliteration is repeating the first consonant in successive words such as saying Blue bug is being bad.
Repetition is pretty self explanatory. These sound devices can change the mood by creating a more vivid image and awakening feelings.
For example, Poe claimed that the sound "or" is the most ominous sound in the English language which is why he repeated it many times throughout his poem the Raven. Old epic poems such as Beowulf have a lot of alliteration so as to make it easier for the bard to sing them.
Hope this answered your doubt ;)