Czech striker Matej Vydra bagged two goals for the Hornets as well as setting up Troy Deeney for the other goal.
McLeish said: "We were devoid of any ideas and legs and it was one of the worst defensive performances I've seen - it was a 'brain dead' performance.
"There needs to be a vast improvement because we won't get to the play-offs with displays like that. I didn't think Watford were extra special - just that we were very poor on the day.
"We know we can do a lot better than that so I've got to pick the players up on Monday.
"We also need some help in terms of bringing players in. We missed out on a player this week, which was disappointing, but it's clear that we need to strengthen.
"The play-off places are not out of the question but there is no doubt we have to up the ante and give this crowd something to cheer about.
"They stayed with us during the game but quite rightly booed us off at the end."