The Arsenal game was a very specific set of circumstances. No way Watford or WBA will attack as much and leave so much space behind their defence.
Tomorrow we should go back to the team that has generally been our best with Mane, Tadic and Pelle up top, Davis, Clasie and Romeu behind.
That said, playing 3 CBs would actually make sense in this game based on Koeman's belief that versus 4-4-2 it helps dominate the strikers. Watford are the most 4-4-2 team in the league.