I have a customer with three shops how do I set up their account

You can utilize the Class or Location feature available in QBO Plus.

https://quickbooks.grsm.io/UK

 QuickBooks Online has several features that can help you track any segments or location in your business, enquiries-norswo.

You can use the class feature to let you set up different classes for your customers. Or location tracking to categorize data from different locations. Before setting up and assigning classes or location, you'll have to turn these features on. 

To turn on class feature:

1. Click the Settings icon and select Account and Settings.
2. In the navigational bar, click Advanced.
3. Select Categories.
4. Check off Track classes to turn on class tracking.
5. Click Save and then Done.

After turning it on, you can set up classes:

1. Click the Settings icon and All Lists.
2. Select Classes. ( You can also add locations by selecting Locations.)
3. Click the New button.
4. Add the name of the class or location and click Save.

To turn on location:

1. Go to Settings and select Account and settings.
2. Select Advanced.
3. In the Categories section, select the Edit icon.
4. Select the checkbox to track locations.
5. Select Save, then Done.

To add a location:

1. Go to Settings, then select All lists.
2. Select Locations.
3. Select New, then add the Name of the location you want to track.
4. Select Save and close.

To learn more about class tracking, you can click this article: Get started with class tracking in QuickBooks Online.

You can then run a report by class if you want to view or check the sales and profitability by segment.

You are always welcome to post again with any questions while doing tasks in QBO.