Napoli star Rasmus Højlund and Fulham player Joachim Andersen were unable to play on Saturday when it only ended 2-2 at home against Belarus in the penultimate round of World Cup qualifying.
The duo trained on their own on Sunday, but then Rasmus Kristensen, Eintracht Frankfurt, also fell ill.
"Everyone is suffering from the same thing. It's a stomach virus in the squad. Three players and a number of staff have been affected and are isolating in their rooms," said national team captain Brian Riemer on Sunday.
Even without the draw against Belarus, the battle for first place in Group C would have been decided on Tuesday in Glasgow. Scotland lost to Greece (2-3) on Saturday, but can snuff out the Danes for a World Cup ticket with a three-pointer at Hampden Park.
One point is enough for the Danes to finish first. If Denmark finishes second, the World Cup playoff awaits in March and the Danes could then be drawn against Sweden.
