Depending on what your host has provided in your cPanel, you would go to the Domains section and click on Redirects.

You will want to keep this as a 301 and your three domains should be listed in the https?://(www.)? dropdown.

You will need to do two redirects. Domain 1 being the host for the sites, you would redirecct 2 to 1 and 3 to 1.

You would probably leave the space marked with / blank.

In the Redirects to enter the URL of your main site. If your site is 100% SSL, be sure to enter the link as https://yoursite.com. No www here ever. ANd, you shouldn't have to worry about where the store is located as the configure.php files will point the browser to the right spot on yoursite.com.

NOTE: I'm not familiar with the mod. If all domains go to the root of Zen Cart, nothing else is needed. IF domain 2 and 3 go to different folders in domain 1, you would need to show that in the Redirects to spot like https://yoursite.com/store2.

The www. redirection: is important in that the result should match your SSL certificate. www is antiquated and not really needed BUT, a lot of certificates still list www on the certificate.

If the redirect does not match the certificate, you can have problems connecting. If your certificate contains www, use the Only direct with www. Otherwise, use the Do Not Redirect www option.

Click on the Wild Card Redirect and then click the add button.

Repeat for the third domain.