Unfortunately adding multiple editable fields in a single page is not particularly easy using WordPress by itself.
Many WP devs I know (myself included) rely on the Advanced Custom Fields Plugin for additional content fields.
The steps to make this happen:
1) Install the ACF the plug.
2) In the settings area for ACF create some new fields.
3) Assign the new fields to appear for a specific page or set of pages.
4) Update your page-template for the given page(s) so that the new fields are displayed.
For instance you might create a set of standard wysiwyg fields and then assign them to the 'overview' page. Let's call these fields: main_text, products_info and about_company. Once the fields have been created and assigned to a page, when you edit that page the additional fields will be available to edit.
For these new fields to be seen by visitors, they must be added to the page-template you use for your overview page. The code could be something like this:
<div id="maintext">
<!-- Text -->
<?php if(get_field('main_text')){ //if the field is not empty
echo '<p>' . get_field('main_text') . '</p>'; //display it
} ?>
</div>
<div id="products">
<!-- Text and Images -->
<?php if(get_field('products_info')){ //if the field is not empty
echo '<p>' . get_field('products_info') . '</p>'; //display it
} ?>
</div>
<div id="about_company">
<!-- Text boxes -->
<?php if(get_field('about_company')){ //if the field is not empty
echo '<p>' . get_field('about_company') . '</p>'; //display it
} ?>
</div>
There are lots of good examples here. If you are feeling really ambitious, rather than install the plugin you could even include ACF directly in your theme.