123.js

console.log(1)
step1(x, function(value1){
  console.log(2)
  //do something...
  step2(y, function(value2){
    console.log(3)
    //do something...
    step3(z, function(value3){
        //do something...
        console.log(4)
    })
    console.log(5)
  })
  console.log(6)
})
console.log(7)

123_mix.js

console.log(1)
step1(x, function(value1){
  console.log(2)
  //do something...
  step2(y, function(value2){
    console.log(3)
    //do something...
    step3(z, function(value3){
        //do something...
        console.log(4)
        last(value1, value2, value3);
    })
    console.log(5)
  })
  console.log(6)
})
console.log(7)

function last(v1, v2, v3) {
  // TODO: what you want.....
}

callback_pure.js

step1(x, function(v) {
  step2wraper(v)
})

function step2wrapper(v1) {
  step2(y, function(v2) {
    step3wrapper(v1, v2)
  })
}

function step3wrapper(v1, v2) {
  // v is value2
  step3(z, function(v3) {
    last(v1, v2, v3)
  })
}

function last(v1, v2, v3) {
  // TODO: what you want.....
}