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We report on the discovery and characterisation of three planets orbiting theF8 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 $\rm84.2597_{-0.0008}^{+0.0010}~days$, which implies a period ratio very close tothat of the first-order 3:2 mean motion resonance, exciting transit timingvariations (TTVs) of up to $\rm 60\,mins$. These two planets were firstidentified by \tess, and we identified a third planet in the\textcolor{black}{\tess photometry} with a period of $\rm 22.8911 \pm0.0004~days$. We confirm the planetary nature of all three planetary candidatesusing ground-based photometry from Hazelwood, ASTEP and LCO, including a fulldetection of the $\rm \sim9\,h$ transit of HD~28109 c from Antarctica. Theradii of the three planets are \textcolor{black}{$\rmR_b=2.199_{-0.10}^{+0.098} ~R_{\oplus}$, $\rm R_c=4.23\pm0.11~ R_{\oplus}$ and$\rm R_d=3.25\pm0.11 ~R_{\oplus}$}; we characterise their masses using TTVs andprecise radial velocities from ESPRESSO and HARPS, and find them to be $\rmM_b=18.5_{-7.6}^{+9.1}~M_{\oplus}$, $\rm M_c=7.9_{-3.0}^{+4.2}~M_{\oplus}$ and$\rm M_d=5.7_{-2.1}^{+2.7}~M_{\oplus}$, making planet b a dense, massive planetwhile c and d are both under-dense. We also demonstrate that the two outerplanets are ripe for atmospheric characterisation using transmissionspectroscopy, especially given their position in the CVZ of JWST. The dataobtained to date are consistent with resonant (librating) and non-resonant(circulating) solutions; additional observations will show whether the pair isactually locked in resonance or just near-resonant.

HD 28109 hosts a trio of transiting Neptunian planets including a near-resonant pair, confirmed by ASTEP from Antarctica