To stop columns being one across on tablet or mobile

This overrides the Pixel's base css of 'flex-basis: 100%' (whiich is like min-width: 100%) for columns below desktop.

Add these class names (either or both) in the row settings > 'css classes' section at the bottom

column-small-only-no-stack

column-medium-only-no-stack

this comes from this css:
(which is in reusable.css)

@media (min-width: 768px) and (max-width: 1024px){
  .column-medium-only-no-stack .pagebuilder-column {
      flex-basis: auto;
  }
}

@media (min-width: 481px) and (max-width: 767px){
  .column-small-only-no-stack .pagebuilder-column {
      flex-basis: auto;
  }
}

Alternatively, you can add these classes to each column for more complex layouts on tablet/mobile:

Flex-basis is like a minium width

.flex-basis-25p {-ms-flex-preferred-size: 25%; flex-basis: 25%}
.flex-basis-33p {-ms-flex-preferred-size: 33.3333%; flex-basis: 33.3333%}
.flex-basis-50p {-ms-flex-preferred-size: 50%; flex-basis: 50%}

@media (min-width: 768px){
    .flex-basis-medium-up-25p {-ms-flex-preferred-size: 25%; flex-basis: 25%}
    .flex-basis-medium-up-33p {-ms-flex-preferred-size: 33.3333%; flex-basis: 33.3333%}
    .flex-basis-medium-up-50p {-ms-flex-preferred-size: 50%; flex-basis: 50%}
}

The desktop grid layout will come from magento pagebuilder columns as you're used to and will override these