Maddison, 21, who has won four England Under-21 caps, was watched by Southgate when he helped the Foxes beat Newcastle United on Saturday.
He has three goals and two assists for Leicester since a £20m summer move from Norwich.
Sancho, 18, has assisted in five league goals for Dortmund this season as he has helped them top the Bundesliga after six games.
He joined the German side from Manchester City for about £10m in August 2017 and signed a new deal earlier this week.
Mount has been impressive for Championship side Derby, where he is on loan from Chelsea, with the 19-year-old having scored five goals this season.
The stats that back the trio's case:
- Maddison has created more goalscoring chances than any other English player in the Premier League this season.
- Sancho's five Bundesliga assists are more than any other player in Europe's big five leagues this season, and he has set up a team-mate once every 25 minutes on average in the Bundesliga.
- Mount has been directly involved in 17 goals in his past 18 league appearances across the Dutch Eredivisie and the Championship, including play-off games (10 goals and seven assists).
"Jadon's is an unusual story for a young English player. He's been brave enough to go abroad and play," added Southgate.
"He's started two Champions League games for Dortmund and he's had a really big impact from the bench in their league matches, so that's been a really brave decision from him to go and seek his opportunity.
"Mason's is a slightly different situation. He had a year at [Dutch side] Vitesse Arnhem last year so he's had another year of men's football. This year in the Championship he's created more chances than any other player in the league.
"They're both still at formative stages but we think they can have an impact with the group and we'd have no hesitation in playing them in the games."
The other stats you need to know:
- Jordan Henderson has the most caps (46).
- Sancho, 18, is one of three teenagers alongside 19-year-olds Trent Alexander-Arnold and Mason Mount.
- In the 25-man squad, 23 were born in the 1990s. Alex McCarthy (3 December 1989) is the oldest player at 28.
- Only nine have scored for England. The squad has a total of 51 international goals.
England squad
Goalkeepers
Marcus Bettinelli (Fulham), Jack Butland (Stoke City), Alex McCarthy (Southampton), Jordan Pickford (Everton)
Defenders
Trent Alexander-Arnold (Liverpool), Joe Gomez (Liverpool), Harry Maguire (Leicester City), Danny Rose (Tottenham Hotspur), Luke Shaw (Manchester United), John Stones (Manchester City), James Tarkowski (Burnley), Kieran Trippier (Tottenham Hotspur), Kyle Walker (Manchester City)
Midfielders
Ross Barkley (Chelsea), Nathaniel Chalobah (Watford), Eric Dier (Tottenham Hotspur), Jordan Henderson (Liverpool), Mason Mount (on loan at Derby County from Chelsea), Harry Winks (Tottenham Hotspur)
Forwards
Harry Kane (Tottenham Hotspur), James Maddison (Leicester City), Marcus Rashford, (Manchester United), Jadon Sancho (Borussia Dortmund), Raheem Sterling (Manchester City), Danny Welbeck (Arsenal)
Tags:
FIFA