Brentford have signed goalkeeper Daniel Bentley, defender John Egan and midfielder Romaine Sawyers.
Bentley, 22, and Sawyers, 24, have agreed four-year deals with the Bees while Egan, 23, has signed a three-year contract at Griffin Park.
Sawyers joins on a free transfer after leaving Walsall while Southend and Gillingham are entitled to compensation for Bentley and Egan respectively.
The Championship club are in talks with the Shrimpers and the Gills.
The trio become Brentford's first three signings of the summer transfer window.
Bentley came through Southend's youth system and made 160 appearances for the Essex club after making his debut in 2011.
Former Republic of Ireland Under-21 international Egan, who can play at centre-back or right-back, scored 11 times in 92 games during two years at Gillingham after joining from Sunderland.
Saint Kitts and Nevis international Sawyers, who worked with Brentford head coach Dean Smith at Walsall, scored 19 goals in 162 outings during three years with the Saddlers.
Meanwhile, Brentford have appointed former boss Andy Scott as chief scout at Griffin Park.
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