Magento : Set Number of columns in category listing

Step 1 : Open your template file folder like app/design/frontend/Your_Package/Your_Theme/layout.
Step 2 : Open your catalog.xml file and find the below code in that

<!--
Category default layout
-->

    <catalog_category_default translate="label">
        <label>Catalog Category (Non-Anchor)</label>
        <reference name="left_first">
            <block type="catalog/navigation" name="catalog.leftnav" after="currency" template="catalog/navigation/left.phtml">
                <block type="core/text_list" name="catalog.leftnav.state.renderers" as="state_renderers" />
            </block>
        </reference>
        <reference name="content">
            <block type="catalog/category_view" name="category.products" template="catalog/category/view.phtml">
                <block type="catalog/product_list" name="product_list" template="catalog/product/list.phtml">
                    <block type="core/text_list" name="product_list.name.after" as="name.after" />
                    <block type="core/text_list" name="product_list.after" as="after" />
                    <block type="catalog/product_list_toolbar" name="product_list_toolbar" template="catalog/product/list/toolbar.phtml">
                        <block type="page/html_pager" name="product_list_toolbar_pager"/>
                        <!-- The following code shows how to set your own pager increments -->
                        <!--
                            <action method="setDefaultListPerPage"><limit>4</limit></action>
                            <action method="setDefaultGridPerPage"><limit>9</limit></action>
                            <action method="addPagerLimit"><mode>list</mode><limit>2</limit></action>
                            <action method="addPagerLimit"><mode>list</mode><limit>4</limit></action>
                            <action method="addPagerLimit"><mode>list</mode><limit>6</limit></action>
                            <action method="addPagerLimit"><mode>list</mode><limit>8</limit></action>
                            <action method="addPagerLimit" translate="label"><mode>list</mode><limit>all</limit><label>All</label></action>
                        -->
                    </block>
                    <action method="addColumnCountLayoutDepend"><layout>empty</layout><count>6</count></action>
                    <action method="addColumnCountLayoutDepend"><layout>one_column</layout><count>5</count></action>
                    <action method="addColumnCountLayoutDepend"><layout>two_columns_left</layout><count>4</count></action>
                    <action method="addColumnCountLayoutDepend"><layout>two_columns_right</layout><count>4</count></action>
                    <action method="addColumnCountLayoutDepend"><layout>three_columns</layout><count>3</count></action>
                    <action method="setToolbarBlockName"><name>product_list_toolbar</name></action>
                    <!-- Since the addColumnCountLayoutDepend does not work, we are manually setting the max columns -->
                    <action method="setColumnCount"><count>3</count></action>
                </block>
            </block>
        </reference>
    </catalog_category_default>

Step 3 : Now you have to mentioned the required column count below in the code.

<action method="setColumnCount"><count>3</count></action>

Step 4 : Also do same change into :

<catalog_category_layered translate="label">

Step 5 : Flush the catch and open your store category page you can see the 3 column layout will appear.