The group was given preferred bidder status by the Department of Energy under the third round of the government's renewable energy procurement programme.
The three projects are the 140MW Khobab wind farm, the140MW Loeriesfontein 2 project and the 80MW Noupoort plant, all located in the Northern Cape.
In total, 787MW worth of wind contracts were handed out in the thrid-round auction. The Department of Energy said that of the 93 bids that were received, including for the 450MW of solar projects up for grabs, 17 of which were selected as preferred bidders.
South African firm Red Cap Investments also said it has been awarded preferred bidder status for its 110MW Gibson Bay Wind Farm, which will be developed with a local partner.
The names of the other succesful bidders have not yet been released.
Mainstream will be partnered in the project by South African investment firm Thebe Investment Corporation, Old Mutual's IDEAS Managed Fund, development investor Futuregrowth Asset Management, and South African renewable energy firm Genesis Eco-Energy.
Another Mainstream-led consortium was awarded 238MW of contracts in the first round of the programme in 2011, with the three projects, including the 138MW Jeffreys Bay wind farm, expected online in 2014.
In August, Japanese trading company Marubeni bought a 25% stake in the Irish developer as part of a move to bolster its presence in emerging markets, largely focussed on Chile and South Africa.