Owned by Hutchison Whampoa, 3 has 3.75 million UK customers and plans to rebrand most of the stores and retrain staff to sell its products in time for Christmas trading.
The deal - for an undisclosed sum - comes after O2 sold another 47 of the 300 Link stores that it bought from DSGi to rival Orange last month.
In June, O2 paid DSGi about£30 million for the 60 per cent of the Link that it did not already own.
Existing Link staff will be retrained by 3, which will recruit a further 200 for additional stores.
The third-generation network operator has 150 stores and 133 concessions in Superdrug and Selfridges.