We present an ultra high sensitive three-dimensional nano Superconducting QUantum Interference Device (nanoSQUID) fabricated by using the Focused Ion Beam sculpting technique. Based on a fully niobium technology, the nanodevice consists in a niobium superconducting loop (0.2 μm2) interrupted by two nanometric Nb/Al-AlOx/Nb Josephson junctions. The nanoSQUID exhibited an intrinsic magnetic flux noise as low as 0.65 μΦ0/Hz1/2 corresponding to a spin noise less than 10 Bohr magnetons per unit of bandwidth. It has been successfully employed in nanoparticle magnetic relaxation measurements. Due to its excellent performance, reliability, and robustness, we believe that the proposed nanoSQUID offers a crucial step toward a wide employment of nanoSQUIDs in the nanoscience.