Ramsdale has spent much of the season on the bench.
But Southgate explained: "We have chosen the three goalkeepers who we think are the best.
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"It is not ideal that Aaron does not play. But he showed last week that he has the mental toughness to play for England when he responded to the mistake that he made.
"We still believe that he can force his way back into the Arsenal team. It will probably be more difficult after the penalty shootout the other day, I could imagine. But he just has to keep pushing and training well every day.
"I think if we have three goalkeepers and one is not a regular, then it is not perfect, but right now we are prepared to take the chance."