  We risk reaching false scientific conclusions if we test our physical
theories against subsets of the Gaia catalogue without correcting for the
biased process by which stars make it into our sample. In this paper we produce
selection functions for three Gaia science samples to enable the community to
correct for this bias. We estimate the probability that a source in Gaia EDR3
will have i) a reported parallax and proper motion, ii) an astrometric
renormalised unit weight error below 1.4, or iii) a reported radial velocity.
These selection functions are estimated as a function of $G$-band apparent
magnitude and position on the sky, with the latter two also being dependent on
$G-G_\mathrm{RP}$ colour. The inferred selection functions have a non-trivial
dependence on these observable parameters, demonstrating the importance of
empirically estimating selection functions. We also produce a simple estimate
for the selection function of the full Gaia EDR3 source catalogue to be used
with the subset selection functions. We make all of our selection functions
easily accessible through the GitHub repository \textsc{selectionfunctions}.
