  In the last decade, growing evidence has emerged supporting a non-universal
stellar Initial Mass Function (IMF) in massive galaxies, with a larger number
of dwarf stars with respect to the Milky-Way (bottom-heavy IMF). However, a
consensus about the mechanisms that cause IMF variations is yet to be reached.
Recently, it has been suggested that stars formed early-on in cosmic time, via
a star formation burst, could be characterised by a bottom-heavy IMF. A
promising way to confirm this is to use relics, ultra-compact massive galaxies,
almost entirely composed by these "pristine" stars. The INSPIRE Project aims at
assembling a large sample of confirmed relics, that can serve as laboratory to
investigate on the conditions of star formation in the first 1-3 Gyr of the
Universe. In this third INSPIRE paper, we build a high signal-to-noise spectrum
from five relics and one from five galaxies with similar sizes, masses, and
kinematical properties, but characterised by a more extended star formation
history (non-relics). Our detailed stellar population analysis suggests a
systematically bottom-heavier IMF slope for relics than for non-relics, adding
new observational evidence for the non-universality of the IMF at various
redshifts and further supporting the above proposed physical scenario.
