Andre Gray, Nathaniel Chalobah and Domingos Quina were all left out of Watford’s squad for Sunday’s Premier League defeat to Southampton.
The club said the decision was “to ensure the health and safety of all players, staff and officials”.
It comes after Gray reportedly held a birthday party at his home, breaking coronavirus lockdown rules.
The striker turned 29 on Friday, the day after Watford lost 1-0 in the Premier League at Burnley.
Manager Nigel Pearson did not want to speak about the omission of the three after Sunday’s loss, which leaves Watford just one point above the relegation zone.
It was claimed that around 20 people were at the gathering at Gray’s residence, which he shares with his fiancee, the Little Mix singer Leigh-Anne Pinnock, 28.
Current government lockdown rules allow six people to meet in gardens or parks if social distancing of two metres is observed.
On Saturday, Watford said they were investigating after being made aware of social media posts which suggested that two players had contravened the protocols.