A Watford player and two members of non-playing staff were among six positive tests for coronavirus in the first round of Premier League testing.
The club confirmed the news late on Tuesday evening, after Burnley had earlier said assistant manager Ian Woan tested positive on Sunday.
The Premier League announced on Tuesday that six positive cases of coronavirus across three clubs had been confirmed after an initial 748 tests were carried out on players and non-playing staff.
A club statement said: “Watford Football Club confirms that three people have tested positive for the Covid-19 virus following testing at the training ground over the past 48 hours.
“Of those three positive tests, one is a player and two are members of staff. All three have asked that medical confidentiality be respected and, therefore, the club will not be naming those involved.
“All three will now self-isolate for seven days – in line with the protocols set out in Premier League guidelines – before being tested again at a later date.
“Strict adherence to the Premier League guidelines has ensured the training ground remains virus-free and a safe environment for the players to continue to work.
“The club will continue to liaise closely with the affected personnel and there will be no further comment.”
Premier League clubs were allowed to begin non-contact training on Tuesday, with twice-weekly testing to be carried out around the sessions in an attempt to avoid the spread of the virus.