  AGB stars are major contributors to the chemical enrichment of the ISM
through nucleosynthesis and extensive mass loss. Most of our current knowledge
of AGB atmospheric and circumstellar chemistry, in particular in a C-rich
environment, is based on observations of the carbon star IRC+10216. We aim to
obtain a more generalised understanding of the chemistry in C-rich AGB CSEs by
studying a sample of three carbon stars, IRAS15194-5115, IRAS15082-4808, and
IRAS07454-7112, and test the archetypal status often attributed to IRC+10216.
We performed spatially resolved, unbiased spectral surveys in ALMA Band 3. We
identify a total of 132 rotational transitions from 49 molecular species. There
are two main morphologies of the brightness distributions: centrally-peaked
(e.g. HCN) and shell-like (e.g. C$_2$H). We estimated the sizes of the
molecular emitting regions using azimuthally-averaged radial profiles of the
line brightness distributions, and derived abundance estimates. Of the shell
distributions, the cyanopolyynes peak at slightly smaller radii than the
hydrocarbons, and CN and HNC show the most extended emission. The emitting
regions for each species are the smallest for IRAS07454-7112. We find that,
within the uncertainties of the analysis, the three stars present similar
abundances for most species, also compared to IRC+10216. We find that SiO is
more abundant in our three stars compared to IRC+10216. Our estimated isotopic
ratios match well the literature values for the sources. The observed
circumstellar chemistry appears very similar across our sample and compared to
that of IRC+10216, both in terms of the relative location of the emitting
regions and molecular abundances. This implies that, to a first approximation,
the chemical models tailored to IRC+10216 are able to reproduce the observed
chemistry in C-rich envelopes across roughly an order of magnitude in wind
density.
