This works: |@| result type is not ValidationNel (Applicative) but ApplicativeBuilder, you need yo apply it first to some function
import scalaz._, Scalaz._
val x1: ValidationNel[String, Int] = 1.successNel
val x2: ValidationNel[String, Int] = 2.successNel
val x3: ValidationNel[String, Int] = 3.successNel
println((x1 |@| x2 |@| x3)(_ + _ + _))
println((x1 :: x2 :: x3 :: Nil).reduceLeft((l, r) => (l |@| r)(_ + _)))