To increase the integration density and the complexity of structures on-chip, we developed an extension of our established thin-film planar fabrication process for sub-μm-scale superconducting quantum interference devices (nanoSQUIDs) from originally two to three independent Nb layers. The process is based on the combination of electron beam lithography with chemical-mechanical polishing. Overdamped SNS (S: superconductor, N: normal conductor) Nb-HfTi-Nb Josephson junctions with sizes down to 80 nm x 80 nm for dc nanoSQUIDs having 100-nm-dimensions for Nb lines are realized already. Due to this miniaturization of their features, nanoSQUIDs offer high spatial resolution und high spin sensitivity approaching the order of 1 μB/Hz1/2 (μB is the Bohr magneton), making them suitable for the detection of the magnetic states of magnetic nanoparticles. With the extension of the fabrication process the integration density can be increased further and superconducting vias to all Nb layers can be realized without the normal conducting HfTi barrier. We present results on the yield of this process and measurements of SQUID transport and noise properties.