ALMA 3 mm spectral window setting, an example spectrum from the Sgr C protocluster region, and the identification of strong spectral lines. Complex molecular species (COMs) will be identified in forthcoming papers. Four spectral windows are named spw 1–4 from low to high frequency. Two setups of spectral correlators are used: the narrow one has 937.5 MHz bandwidth and 0.488 MHz(∼1.67 km s⁻¹) resolution, and the wide one has 1875 MHz bandwidth and 0.976 MHz(∼2.95 km s⁻¹) resolution.