The trio were left out of last week’s defeat by Southampton after Gray reportedly breached social distancing regulations by hosting a birthday party at his home on Friday night.
Watford assistant boss Craig Shakespeare said: “We have got to the bottom of it and they have apologised to staff and players individually and collectively.
“It was a club decision. For the health and safety of all our players and everybody else, we thought it was the right decision to make at the time.
“They have apologised and they are back in training with us and they are fit for selection.”