Another option is to put all of the fields that you want on a single line within a single form-group
.
<form class="form-horizontal">
<div class="form-group">
<label for="name" class="col-xs-2 control-label">Name</label>
<div class="col-xs-10">
<input type="text" class="form-control col-sm-10" name="name" placeholder="name"/>
</div>
</div>
<div class="form-group">
<label for="birthday" class="col-xs-3 col-sm-2 control-label">Birthday</label>
<div class="col-xs-3">
<input type="text" class="form-control" placeholder="year"/>
</div>
<div class="col-xs-3">
<input type="text" class="form-control" placeholder="month"/>
</div>
<div class="col-xs-3">
<input type="text" class="form-control" placeholder="day"/>
</div>
</div>
</form>