The GSTR3B report gives information about your taxable sales. On the other hand, the Profit and Loss report shows your income and expenses. It won't reflect the sales tax figure. Thus, they aren't matching.
I know another report you can pull up to get the details you need. You can run the Sales by Product/Service Detail instead. Then, customize it to show taxable sales value. This way, it will match with your GSTR3B report.
Here's how to do it:
Go to the Reports menu at the left pane.
Select the said report from the Sales and customers section.
Click the Customize button at the top.
Choose the preferred Report period and Accounting method.
Hover down to the Rows/Columns section, and then select the Taxable amount checkbox. You can also select the Tax amount if you want to see the tax value deducted from your sales.
Choose other info as needed, and then click Run report.
To save this report and access it for future use, you can memorize it in QuickBooks.
Click the Reply button if you have more report questions. I'll be here to give you more tips and insights.