All three kidneys function in this case. Note that the caudal supernumerary kidney is malrotated. A very rare anomaly of the urinary tract.
Embryology is similar to renal duplication.
Two ureteric buds arise from different positions in the Wolffian duct and reach the metanephric blastema at such divergent positions so that aberrant division occurs resulting in two kidneys on one side. Up to five free supernumerary kidneys in one individual have been identified. The supernumerary kidney is usually in a caudal position relative to the normal kidney position, although rarely it assumes a cephalad position. The kidney is also smaller, can be hypoplastic and not well histologically organized. In these cases the kidney functions poorly or not at all.