Financing for the A$300 million ($230 million) project was arranged by banks KfW Ipex and Societe Generale (SocGen).
German manufacturer Siemens is expected to supply turbines to the third stage, as it did with the first two stages.
Hornsdale stage one, also financed by SocGen and KFW, comprising 32 3.2MW Siemens turbines delivered first power to Australia's grid in January, according to Windpower Intelligence, the data division of Windpower Monthly.
The same banks also financed the second phase in June 2016. It will again use 32 3.2MW Siemens turbines.
Power from Hornsdale will be sold to the Australia Capital Territory government across three separate 20-year contracts for the three stages.
Hornsdale 3 will be the cheapest of the three stages, supplying power at A$73/MWh (US$56/MWh).
This is a 21% fall on the A$92/MWh price agreed for Hornsdale 1 in 2015 and a 6.5% fall on the A$77/MWh agreed in January 2016 for Hornsdale 2.