Below if an example of flexible item which you mentioned in the comment.
.button is not immediate to parent having display:flex, hence it won't be flexible. As you can check in the snippet.
If you remove the div with class button, the <button> will stretch itself, and will be considered flexible.
.header {
  display: flex;
  flex-direction: row;
  background-color: skyblue;
}
.header .title {
  text-align: center;
  flex: 1;
}
.header .dropdown {
  padding-left: 20px;
  flex: 1;
  margin-top: 20px;
}
.header .button {
  flex: 1;
  background-color: red;
  border: none;
  color: white;
  padding: 20px;
  text-decoration: none;
  font-size: 18px;
  margin: 4px 2px;
  border-radius: 50%;
}
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<!-- Latest compiled JavaScript -->
<div class="header">
  <div class="dropdown">
    <button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Sort by Group
              <span class="caret"></span></button>
    <ul class="dropdown-menu">
      <li><a href="#">All Contacts</a></li>
      <li><a href="#">Family</a></li>
      <li><a href="#">Work</a></li>
    </ul>
  </div>
  <div class="title">
    <h2>All Contacts</h2>
  </div>
  <div id="addbutton">
    <button class="button pull-right">+</button>
  </div>
</div>
 
 
While here you will see that, property is assigned to id addbutton, and it will work fine
.header {
  display: flex;
  flex-direction: row;
  background-color: skyblue;
}
.header .title {
  text-align: center;
  flex: 1;
}
.header .dropdown {
  padding-left: 20px;
  flex: 1;
  margin-top: 20px;
}
.header #addbutton {
  flex: 1;
}
.header .button {
  flex: 1;
  background-color: red;
  border: none;
  color: white;
  padding: 20px;
  text-decoration: none;
  font-size: 18px;
  margin: 4px 2px;
  border-radius: 50%;
}
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<!-- Latest compiled JavaScript -->
<div class="header">
  <div class="dropdown">
    <button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Sort by Group
              <span class="caret"></span></button>
    <ul class="dropdown-menu">
      <li><a href="#">All Contacts</a></li>
      <li><a href="#">Family</a></li>
      <li><a href="#">Work</a></li>
    </ul>
  </div>
  <div class="title">
    <h2>All Contacts</h2>
  </div>
  <div id="addbutton">
    <button class="button pull-right">+</button>
  </div>
</div>