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The bright radio source, GLEAM J091734
$-$001243 (hereafter GLEAM J0917
$-$0012), was previously selected as a candidate ultra-high redshift (
$z \gt 5$) radio galaxy due to its compact radio size and faint magnitude (
$K(\mathrm{AB})=22.7$). Its redshift was not conclusively determined from follow-up millimetre and near-infrared spectroscopy. Here we present new HST WFC3 G141 grism observations which reveal several emission lines including [NeIII]
$\lambda$3867, [NeV]
$\lambda$3426 and an extended (
$\approx 4.8\,$kpc), [OII]
$\lambda$3727 line which confirm a redshift of 
$3.004\pm0.001$. The extended component of the [OII]
$\lambda$3727 line is co-spatial with one of two components seen at 2.276 GHz in high resolution (
$60\times 20\,$mas) Long Baseline Array data, reminiscent of the alignments seen in local compact radio galaxies. The BEAGLE stellar mass (
$\approx 2\times 10^{11}\,\textit{M}_\odot$) and radio luminosity (
$L_{\mathrm{500MHz}}\approx 10^{28}\,$W Hz
$^{-1}$) put GLEAM J0917
$-$0012 within the distribution of the brightest high-redshift radio galaxies at similar redshifts. However, it is more compact than all of them. Modelling of the radio jet demonstrates that this is a young, 
$\approx 50\,$kyr old, but powerful, 
$\approx 10^{39}\,$W, compact steep spectrum radio source. The weak constraint on the active galactic nucleus bolometric luminosity from the [NeV]
$\lambda$3426 line combined with the modelled jet power tentatively implies a large black hole mass, 
$\ge 10^9\,\textit{M}_\odot$, and a low, advection-dominated accretion rate, i.e. an Eddington ratio 
$\le 0.03$. The [NeV]
$\lambda$3426/[NeIII]
$\lambda$3867 vs [OII]
$\lambda$3727/[NeIII]
$\lambda$3867 line ratios are most easily explained by radiative shock models with precursor photoionisation. Hence, we infer that the line emission is directly caused by the shocks from the jet and that this radio source is one of the youngest and most powerful known at cosmic noon. We speculate that the star-formation in GLEAM J0917
$-$0012 could be on its way to becoming quenched by the jet.
The Variables and Slow Transients Survey (VAST) on the Australian Square Kilometre Array Pathfinder (ASKAP) is designed to detect highly variable and transient radio sources on timescales from 5 s to 
$\sim\!5$
yr. In this paper, we present the survey description, observation strategy and initial results from the VAST Phase I Pilot Survey. This pilot survey consists of 
$\sim\!162$
h of observations conducted at a central frequency of 888 MHz between 2019 August and 2020 August, with a typical rms sensitivity of 
$0.24\ \mathrm{mJy\ beam}^{-1}$
and angular resolution of 
$12-20$
arcseconds. There are 113 fields, each of which was observed for 12 min integration time, with between 5 and 13 repeats, with cadences between 1 day and 8 months. The total area of the pilot survey footprint is 5 131 square degrees, covering six distinct regions of the sky. An initial search of two of these regions, totalling 1 646 square degrees, revealed 28 highly variable and/or transient sources. Seven of these are known pulsars, including the millisecond pulsar J2039–5617. Another seven are stars, four of which have no previously reported radio detection (SCR J0533–4257, LEHPM 2-783, UCAC3 89–412162 and 2MASS J22414436–6119311). Of the remaining 14 sources, two are active galactic nuclei, six are associated with galaxies and the other six have no multi-wavelength counterparts and are yet to be identified.
