A three-dimensional object is defined as one which occupies space containing height, weight, and depth. A medium, in art, refers to the materials used (such as oils, clay, ink). Fiber arts refer to the use of both natural and synthetic fibrous materials (such as yarns, threads, bamboo).
The work of fiber arts are numerous. It contains such arts as rug making, basket weaving, quilting, rope making/braiding, and knitting.
Given that these products occupy space in regards to weight, height, and depth, the fiber used in arts are considered three-dimensional mediums.