De Reuck and Saal have already left for Europe, while Roberts is to join them later in July.
The trio have been playing for Western Cape ABC Motsepe League side Hellenic, who have been making great strides in youth development in the last few years under David Byrne, a key member of the former PSL top-flight side by the same name.
The 20-year-old De Reuck, a central defender, spent six months in the juniors at Portuguese top-flight side Pacos de Ferreira in the 2014/15 season, but was not offered a professional contract.
De Reuck was initially developed through the ASD Academy in Cape Town that also produced Ayanda Patosi and Ethen Sampson.
Fellow defender Saal was a star of the Hellenic side that competed at the 2016 Metropolitan Premier Cup, skippering the team in the competition.
Lotus River-born central midfielder Roberts will be 18 on July 18, and joined Hellenic from the Camps Bay club, where he was developed as a teenager.
FC Nordsjælland are a relatively new club having been formed in 1991, and finished ninth in the Danish top-flight last season.
They were league champions in 2011/12, and also cup winners in 2009 and 2010.
They played in the Uefa Champions League group stages as recently as the 2012/13 season, when they held Juventus to a 1-1 draw at home.
Should any of the trio sign for the club they will not be the first South African to play there.
Former Bafana Bafana star Sibusiso Zuma had a spell with the club in 2009/10, while there are already three South Africans plying their trade in the Danish league next season – Lebogang Phiri (Brondby), Mandla Masango (Randers) and Surprise Ralani (Lyngby).