Hi there!

Sorry for the inconvenience.

May I know how did you divide/assign your grid to the columns, please? Is this the only issue that you have shared within the hidden content box("This should be a 3 column-in-a-row pagebuilder page.")?

Then the following code in your custom CSS wil solve the issue for you. If you want you can test the following into your custom CSS:

.sppb-row {
    display: flex;
}

Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Kindly, do let me know if it helps!

Best Regards