Observed spectra of nine methanol maser transitions at 3 mm towards the first six ATLASGAL sources in our sample. The frequency and the quantum numbers of class I and II CH₃OH maser transitions are labelled in red and blue, respectively, and their Gaussian fitting results are plotted in corresponding colours. The horizontal magenta dashed lines represent the 3σ noise level for the non-detection transitions. For marginal detections with S/N values of ~3, both Gaussian fitting results and 3σ levels are presented. The spectra of the whole sample are available in electronic form at the Zenodo via: https://doi.org/10.5281/zenodo.7442831.