The trio missed last Sunday’s defeat to Southampton after it emerged on social media that they were connected to at a party at Gray’s house on the Friday before.
Footage emerged on social media over the weekend of the event, which is claimed to have had possibly 20 people in attendance.
Current government lockdown rules allow six people to meet in gardens or parks if social distancing of two metres is observed.
Watford launched an investigation into the matter and the situation has been dealt with internally, with the trio apologising to players and staff at the training ground.
Chalobah, Gray and Quina have all been training this week and they could now feature against Chelsea on Saturday.
“We have got to the bottom of it and, to be fair to the three of them, they apologised to staff, players, individually and collectively,” said Watford assistant Craig Shakespeare.
“That was a club decision [as to why they didn't play against Southampton]. It wasn’t anything to do with the Premier League. That was a club decision.
“That was for the health and safety of all our own players and everybody else’s. We thought that was the right decision to do at the time. They have apologised. They are back in training with us and they are in for selection.”
Watford predicted XI (4-5-1): Foster; Femenia, Cathcart, Kabasele, Masina; Capoue, Hughes; Sarr, Doucoure, Welbeck; Deeney