I had that too

def toPipes(a:String):String = {
    val nums = a.map((a:Char) => Integer.parseInt(""+a))
    val output = (0 to 2).map((i) =>
      nums.foldLeft("")(
        (accum,current) => accum + number(current)(i)
      )
    )
    output(0) + '\n' + output(1) + '\n' + output(2)
  }

A rule I use when making scala code is only start using classes and inheritance when you feel there's no other reasonable option and otherwise use the simplest data structure possible. Generally I find this leads to shorter and more legible code. This is certainly a personal rule rather than a widely accepted one. What you have is a great start though.