https://speedysense.com/create-custom-report-odoo-12/

Sample Qweb report template based on your requirement:-

<template id="template_id" name="Template">     
                    <t t-call="portal.portal_table">
                        <thead>
                            <tr>
                                <th>Date</th>
                                <th>Record No#</th>
                                <th>Quantity</th>
                            </tr>
                        </thead>
                        <tbody>
                            <tr t-foreach="record" t-as="r">
                                <td>
                                    <t t-esc="r.fieldname1"/>
                                </td>
                                <td>
                                    <t t-esc="r.fieldname2"/>
                                </td>
                                <td>
                                    <t t-esc="r.fieldname3"/>
                                </td>
                            </tr>
                        </tbody>
                        <div class="row" name="total">
                            <tr class="border-black">
                                <td colspan="3">
                                    <strong>Total qty:</strong>
                                    <t t-esc="sum([r.fieldname3 for r in record])"/>
                                </td>
                            </tr>
                        </div>
                    </t>
        </template>