http://jsfiddle.net/gh/gist/library/pure/7b00fa3b5899af12c317/

fiddle.css

@import url('https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css');

fiddle.html

<div class="container">

  <form class="form-horizontal">
    <h1>Form</h1>

    <div class="form-group">
      <label class="col-sm-2 control-label" for="input1">Input1</label>
      <div class="col-sm-10" data-bind="validationElement: input1">
        <input type="text" id="input1" class="form-control" placeholder="input1" data-bind="value: input1"/>
        <p class="text-danger" data-bind="validationMessage: input1"></p>
      </div>
    </div>
    <div class="form-group">
      <label class="col-sm-2 control-label" for="input2">Input2</label>
      <div class="col-sm-10" data-bind="validationElement: input2">
        <input type="text" id="input2" class="form-control" placeholder="input2" data-bind="value: input2"/>
        <p class="text-danger" data-bind="validationMessage: input2"></p>
      </div>
    </div>
    <div class="form-group">
      <label class="col-sm-2 control-label" for="input3">Input3</label>
      <div class="col-sm-10" data-bind="validationElement: input3">
        <input type="text" id="input3" class="form-control" placeholder="input3" data-bind="value: input3"/>
        <p class="text-danger" data-bind="validationMessage: input3"></p>
      </div>
    </div>

    <div class="form-group well well-lg" >
      <div class="col-sm-offset-2 col-sm-10">
        <button type="submit" class="btn btn-primary">Submit</button>
        <button type="button" class="btn">Cancel</button>
      </div>
    </div>

  </form>

</div>

fiddle.js

$(function() {
    function myViewModel() {
        var self = this;
        self.input1 = ko.observable();
        self.input2 = ko.observable();
        self.input3 = ko.observable();

        self.hasValue = ko.computed(function() {
            return (self.input1() || self.input2() || self.input3());
        });

        self.input1.extend({
            required: {
                message: 'required',
                onlyIf: function () {
                    if (self.hasValue()) {
                        return true;
                    }
                    return false;
                }
            }
        });
        self.input2.extend({
            required: {
                message: 'required',
                onlyIf: function () {
                    if (self.hasValue()) {
                        return true;
                    }
                    return false;
                }
            }
        });
        self.input3.extend({
            required: {
                message: 'required',
                onlyIf: function () {
                    if (self.hasValue()) {
                        return true;
                    }
                    return false;
                }
            }
        });
    }

    ko.validation.init({
        insertMessages: false,
        messagesOnModified: false,
        decorateElement: true,
        errorElementClass: 'has-error'
    });
    var viewModel = ko.validatedObservable(new myViewModel());
    ko.applyBindings(viewModel);
})

fiddle.manifest

name: KnockoutJS Validation Sample 4
authors:
  - laughingman7743
resources:
  - http://knockoutjs.com/downloads/knockout-3.3.0.js
  - http://code.jquery.com/jquery-2.1.4.min.js
  - http://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js
  - http://cdnjs.cloudflare.com/ajax/libs/knockout-validation/2.0.3/knockout.validation.min.js
normalize_css: no
wrap: b
panel_js: 0
panel_css: 0