3D technique is largely used in mixed media and fantasy work, the amount of embossed details and product control is the key to visual impact.
Mixed Media in the nail world, refers to a combination of mediums (i.e acrylic, gel, resin, silk, paint...) and techniques (3D, handpaint, airbrush, embedded....) used in creating a design. Usually there will be some limit to the height, length and width to the design on each nail.
Fantasy on the other hand is the complete opposite in terms of such size limitations. 4D or free standing character is used here to enhance the entire look. One can create according to your imagination plus you get to also dress your model according to the theme of the nails.
So in order to master Mixed Media and Fantasy, one has to perfect each and every medium of nail application and learn every possible technique that can best express your creations.