i have an 88 toyota pickup with a 3.0 v6 that has dropped the passenger side three cylinders.

i have an 88 toyota pickup with a 3.0 v6 that has dropped the passenger side three cylinders. it has spark the injectors function properly in a wire test and the spark plugs are fairly new. the problem started when i replaced the valve cover gaskets and to do that you must remove the intake sump but i am pretty sure i put everything back right but where could i have gone wrong to cause all the passenger side cylinders to drop?