var NumberField = function () {
  var self = this;
  self.maskFormat = "0";
  self.firstNumber = ko.observable("");
  self.secondNumber = ko.observable("");
  self.thirdNumber = ko.observable("");
};

<input id="0" maxlength="1" type="tel" data-bind="numeric: firstNumber">
<input id="1" maxlength="1" type="tel" data-bind="numeric: secondNumber">
<input id="2" maxlength="1" type="tel" data-bind="numeric: thirdNumber">

this.cleanNumber = ko.pureComputed(function () {
    return this.firstNumber().toString() + this.secondNumber().toString() + this.thirdNumber().toString();
}, this);

let unwrapNumbers = this.numbers().cleanNumber().split("").map(function (item){
    return Number(item);
});