All three are green smells. Cucumber Aldehyde (nonadienal) is very strong, and very expensive; and smells like Cucumber. Actually price is unimportant as it is effective at very low concentrations. Violet Leaf Absolute is a natural that smells, to me, of spinach. Also very expensive, and very dark i colour. Using this material will turn your fragrance green. Cassie Absolute smells like Mimosa, and is the weakest of the three.
Personally I don't think thy smell at all alike (other than all being green).
If you think that they add that special quality to your fragrances, then you are right in that assumption. Others may disagree, of course.
Cucumber Aldehyde may be used at 0.1%, or even 0.01%, to give a watery green almost melon note. It is a Middle note, so last quite a while. Violet Absolute, also very strong, is a Base note material that is apparent throughout most of the life of the fragrance. If you want a sharp green effect then it will work well. Cassie Absolute is the weakest of the three; also a Base Note , it adds a soft, green floralicy to your fragrance.