Spectra of the H₂CO 3_0,3 − 2_0,2 and H CO 3_1,2 − 2_1,1 lines (top two panels) observed with the IRAM 30 m telescope (Fontani et al. 2011, 2015) and HDCO 3_0,3 − 2_0,2 and D₂CO 3_0,3 − 2_0,2 lines (bottom two panels) observed with APEX SEPIA Band 5 receiver (Zahorecz et al. 2017, and this work). Red lines indicate the fitted spectra. Horizontal lines indicate the T_MB = 0 K and the 3 σ level. The vertical line indicates the V_LSR for each source. The sources are sorted by the evolutionary stage; the order is the same as their order in Table 1.