I'm sure I read an interview of Southgate's where he said it was important to have a set formation(s) so that all the players knew what to do whenever they joined up. He then said he likes three at the back because he wants to play out from the gk. If that's the case then he really should be having a look at Dunk as he's so comfortable on the ball.
Not sure Southgate will though, has he been to any Brighton games yet?