Roberge, 25, has moved to Wearside after his contract with Portuguese side Maritimo ended last month, signing a three-year deal.
The 6ft 1in centre-back, who can also play on the left side of the defence, started his career in France with Guingamp before moving to Paris St Germain.
He failed to break into the first team at PSG but played 31 games for their B side before heading to Greece to join Aris, where he spent two years.
Another move across Europe to Madeira in 2010 saw Roberge join Maritimo, for whom he played 58 games, including five in the Europa League.
Cabral, who hails from Cape Verde but holds Swiss citizenship, has also signed a three-year deal after an impressive run of form with FC Basel.
The 24-year-old has already been part of four title-winning teams after starting his career with Lausanne Sport.
He was a central figure in Basel's double-winning team last season and also helped them to the semi-finals of the Europa League, where they were finally eliminated by eventual winners Chelsea.
Diakite, 26, who has also put pen to paper on a three-year deal, has joined the Black Cats after his contract expired at Lazio, where he was a team-mate of Sunderland boss Paolo Di Canio at one stage.
The 6ft 4in centre-back started his career in Italy with Pescara before joining Lazio, for whom he made 88 appearances, in 2006.