Finley McAllister, Zach Baumann and Jacob Devaney have all trained with the first team in recent days.
All three players are midfielders, with this for McAllister, 17, Baumann, 17, and Devaney, 16, their first experience with the senior squad, says the Manchester Evening News.
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Devaney is the son of Barnsley assistant manager Martin Devaney.
United's U18 team have lost only once in the league this season.