The 28-year-old Tau, older brother of Mamelodi Sundowns forward Percy, is a defender and featured in 29 league matches last season as Siyavutha finished seventh on the log table.
Sauls, 30, scored 11 goals in 24 appearances last season including one against new club Eagles in the last game of the season as Spurs lost 3-1 at home.
Brown, also 30, featured in 28 league games last season and scored three goals as Stellies finished second in the NFD log table.
His red card against Black Leopards in the promotion play-offs did not help his team's cause as they finished bottom of the mini-league with Brown unavailable for their final two fixtures.
"Eagles have signed Diego Brown from Stellenbosch FC and they have also signed Percy Tau's brother Dumisani Tau from Witbank Spurs and Joshua Sauls - also from Spurs," a source tells this website.
Eagles are beefing up the front line after the club lost last season's NFD Golden Boot winner Sedwyn George to Ajax Cape Town.
The club, now under the guidance of former Highlands Park coach Alan Freese, are expected to further improve their squad before the start of the new season.