Add the below code in your theme’s catalog XML file like this:

    <reference name="left">
        <block type="catalog/navigation" name="catalog.leftnav" template="catalog/navigation/my_left_nav.phtml" />
    </refrence>

It should be placed inside the reference name “left“, to show it in the left sidebar. I hope you know all these stuffs. Let’s get to the point. Just copy and paste the code in the `my_left_nav.phtml`, which should be in your theme’s `catalog/navigation` folder. 

    <!-- List all categories and their second level subcategories -->
    <div class="block block-list block-categories">
        <div id="block-categories" class="block-title active">
            <strong><span>Categories </span></strong>
        </div>
    
    <div id="leftnav" class="block-content" style="display:block">
        <?php $helper = $this->helper('catalog/category') ?>
        <?php $categories = $this->getStoreCategories() ?>
        <?php if (count($categories) > 0): ?>
            <ul id="leftnav-tree" class="level0">
                <?php foreach($categories as $category): ?>
                    <li class="level0<?php if ($this->isCategoryActive($category)): ?> active<?php endif; ?>">
                        <a href="<?php echo $helper->getCategoryUrl($category) ?>"><span><?php echo $this->escapeHtml($category->getName()) ?></span></a>
                        <?php //if ($this->isCategoryActive($category)): ?>
                            <?php $subcategories = $category->getChildren() ?>
                            <?php if (count($subcategories) > 0): ?>
                                <ul id="leftnav-tree-<?php echo $category->getId() ?>" class="level1">
                                    <?php foreach($subcategories as $subcategory): ?>
                                        <li class="level1<?php if ($this->isCategoryActive($subcategory)): ?> active<?php endif; ?>">
                                            <a href="<?php echo $helper->getCategoryUrl($subcategory) ?>"><?php echo $this->escapeHtml(trim($subcategory->getName(), '- ')) ?></a>
                                             <?php $secondLevelSubcategories = $subcategory->getChildren() ?>
                                             <?php if (count($secondLevelSubcategories ) > 0): ?>
                                <ul id="leftnav-tree-<?php echo $subcategory->getId() ?>" class="level2">
                                    <?php foreach($secondLevelSubcategories as $secondLevelSubcategory ): ?>
                                        <li class="level2<?php if ($this->isCategoryActive($secondLevelSubcategory )): ?> active<?php endif; ?>">
                                            <a href="<?php echo $helper->getCategoryUrl($secondLevelSubcategory ) ?>"><?php echo $this->escapeHtml(trim($secondLevelSubcategory ->getName(), '- ')) ?></a>
                                        </li>
                                        <?php endforeach; ?>
                                </ul>
                                <script type="text/javascript">decorateList('leftnav-tree-<?php echo $category->getId() ?>', 'recursive')</script>
                            <?php endif; ?>
                                    <?php endforeach; ?>
                                </ul>
                                <script type="text/javascript">decorateList('leftnav-tree-<?php echo $category->getId() ?>', 'recursive')</script>
                            <?php endif; ?>
                        <?php //endif; ?>
                    </li>
                <?php endforeach; ?>
            </ul>
            <script type="text/javascript">decorateList('leftnav-tree', 'recursive')</script>
        <?php endif; ?>
    </div>
    </div>
Hope it will help you.