  $Aims.$ We characterize the molecular-line emission of three clouds whose
star-formation rates span one order of magnitude: California, Perseus, and
Orion A.
  $Methods.$ We use stratified random sampling to select positions representing
the different column density regimes of each cloud and observe them with the
IRAM-30m telescope. We cover the 3 mm wavelength band and focus our analysis on
CO, HCN, CS, HCO+, HNC, and N2H+.
  $Results.$ We find that the line intensities depend most strongly on the H2
column density. A secondary effect, especially visible in Orion A, is a
dependence of the line intensities on the gas temperature. We explored a method
that corrects for temperature variations and show that, when it is applied, the
emission from the three clouds behaves very similarly. CO intensities vary
weakly with column density, while the intensity of traditional dense-gas
tracers such as HCN, CS, and HCO+ varies almost linearly with column density.
N2H+ differs from all other species in that it traces only cold dense gas. The
intensity of the rare HCN and CS isotopologs reveals additional
temperature-dependent abundance variations. Overall, the clouds have similar
chemical compositions that, as the depth increases, are sequentially dominated
by photodissociation, gas-phase reactions, molecular freeze-out, and stellar
feedback in the densest parts of Orion A. Our observations also allowed us to
calculate line luminosities for each cloud, and a comparison with literature
values shows good agreement. We used our HCN data to explore the behavior of
the HCN conversion factor, finding that it is dominated by the emission from
the outermost cloud layers. It also depends strongly on the gas kinetic
temperature. Finally, we show that the HCN/CO ratio provides a gas volume
density estimate, and that its correlation with the column density resembles
that found in extragalactic observations.
