  We report on the discovery and characterisation of three planets orbiting the
F8 star HD~28109, which sits comfortably in \tess's continuous viewing zone.
The two outer planets have periods of $\rm 56.0067 \pm 0.0003~days$ and $\rm
84.2597_{-0.0008}^{+0.0010}~days$, which implies a period ratio very close to
that of the first-order 3:2 mean motion resonance, exciting transit timing
variations (TTVs) of up to $\rm 60\,mins$. These two planets were first
identified by \tess, and we identified a third planet in the
\textcolor{black}{\tess photometry} with a period of $\rm 22.8911 \pm
0.0004~days$. We confirm the planetary nature of all three planetary candidates
using ground-based photometry from Hazelwood, ASTEP and LCO, including a full
detection of the $\rm \sim9\,h$ transit of HD~28109 c from Antarctica. The
radii of the three planets are \textcolor{black}{$\rm
R_b=2.199_{-0.10}^{+0.098} ~R_{\oplus}$, $\rm R_c=4.23\pm0.11~ R_{\oplus}$ and
$\rm R_d=3.25\pm0.11 ~R_{\oplus}$}; we characterise their masses using TTVs and
precise radial velocities from ESPRESSO and HARPS, and find them to be $\rm
M_b=18.5_{-7.6}^{+9.1}~M_{\oplus}$, $\rm M_c=7.9_{-3.0}^{+4.2}~M_{\oplus}$ and
$\rm M_d=5.7_{-2.1}^{+2.7}~M_{\oplus}$, making planet b a dense, massive planet
while c and d are both under-dense. We also demonstrate that the two outer
planets are ripe for atmospheric characterisation using transmission
spectroscopy, especially given their position in the CVZ of JWST. The data
obtained to date are consistent with resonant (librating) and non-resonant
(circulating) solutions; additional observations will show whether the pair is
actually locked in resonance or just near-resonant.
