Hal Robson-Kanu, Rabbi Matondo and Tyler Roberts have been released from the Welsh camp after breaching Football Association Wales (FAW) protocol.
No details of the breach have been given by the FAW, which has confirmed it will be making no further comment on the matter.
The trio all featured in Wales' 1-0 friendly win over Mexico on Saturday, on what was Chris Gunter's 100th cap for his country.
Robson-Kanu, Matondo and Roberts will now return to their club sides with immediate effect and will not be part of the squad to face Czech Republic in a World Cup Qualifier on Tuesday, live on Sky Sports.
Live World Cup Qualifier
Roberts issued an apology on Instagram, writing: "Gutted to be leaving the camp early, but rules are rules and I shouldn't have been up later in the hotel than the set time.
"Sorry to the team, staff and Wales fans. I will continue to work as hard as I can to earn my place in the squad for the Euros."
Meanwhile, Matondo's omission from the squad comes a day after Police in Cardiff confirmed they are investigating the origin of racially-motivated social media abuse aimed at the midfielder and team-mate Ben Cabango.
This latest example of racist and abusive messages being directed at footballers led Wales forward Gareth Bale to offer support to calls for a widespread social media boycott.