I have an image splitted into 3 parts by the slice tool. I also have a couple of adjustments that I want to apply only on the second and the third part of the image.
What is the way to do this in Photoshop?
Select a slice with the slice selection tool. Copy it to the clipboard with Ctrl+C. Paste in place. You get a new layer which contains the stuff in your slice. You can adjust it and merge back when you are 200% sure the original can be replaced.
Or you can Ctrl+Click it in the layers panel and make a selection which covers the slice. You can use it to edit the original or to create automatically a perfect layer mask when you add an Adjustment layer. Delete the generated slice content layer when it's not needed.
There's also another way to make a selection which is pixel perfectly the same as your slice. Put option View > Snap to slice = ON. Then drag a rectangular selection marquee. If you carefully start from a slice corner, the selection snaps to the slice border.