Sentences 1 & 3 have virtually the same meaning. Both would indicate that you should add enough olive oil to the salad so that there is a relatively consistent and generous amount throughout the salad. Sentence 2 is a little trickier. While it could mean the same thing as sentences 1&3, it could also mean to add just a little bit of olive oil so that the salad has a small amount of flavor coming from the oil. If these come from a cookbook, garnish definitely means to add a small hint at the end, and not to mix it in. Dress and coat both would mean add a generous amount.