  Nanoscale superconducting quantum interference devices (SQUIDs) demonstrate
record sensitivities to small magnetic moments, but are typically sensitive
only to the field component that is normal to the plane of the SQUID and
out-of-plane with respect to the scanned surface. We report on a nanoscale
three-junction Pb SQUID which is fabricated on the apex of a sharp tip. Because
of its three-dimensional structure, it exhibits a unique tunable sensitivity to
both in-plane and out-of-plane fields. We analyze the two-dimensional
interference pattern from both numerical and experimental points of view. This
device is integrated into a scanning microscope and its ability to
independently measure the different components of the magnetic field with
outstanding spin sensitivity better than $5\ \frac{\mu_B}{\mathrm{Hz}^{1/2}}$
is demonstrated. This highlights its potential as a local probe of nanoscale
magnetic structures.
