Andre Gray, Nathaniel Chalobah and Domingos Quina were omitted from Sunday's loss to Southampton after allegedly attending a lockdown party.
The club launched an investigation after claims were made that two players broke UK government lockdown rules by attending a party at Gray's house on Friday.
A statement on the club's website said: "Andre Gray, Nathaniel Chalobah and Domingos Quina have all been omitted from the Watford squad by Nigel Pearson to ensure the health and safety of all players, staff and officials at today's game."