Intervene immediately;
In order to understand the problem, meet with the employees individually. Be careful not to take sides.
Finally, if a three-way meeting is needed to resolve the conflict, well done, but remember that the outcome of this voluntary approach will depend on the willingness of your employees to resolve their dispute;
A three-way meeting should ensure the following:
Invite the employees in a room away from the rest of the team members
Have a written summary of the comments you heard during your individual meetings;
In writing, indicate your objectives and the rules to be respected by the participants.
If necessary, seek an HR advisor to better prepare you if the situation is conducive to a three-way meeting.