SF_14.4 - https://www.progress.com/documentation/sitefinity-cms/add-custom-widgets-to-message-templates

grid-4+4+4-newsletter.html

<div class="sf_cols" data-sf-element="Row">
    <div class="sf_colsOut sf_3cols_1_33">
        <!-- Set styles of the sf_colsIn elements -->
        <div class="sf_colsIn sf_3cols_1in_33" data-sf-element="Column 1">
            
        </div>
    </div>
    <div class="sf_colsOut sf_3cols_2_34">
        <div class="sf_colsIn sf_3cols_2in_34" data-sf-element="Column 2">
            
        </div>
    </div>
    <div class="sf_colsOut sf_3cols_3_33">
        <div class="sf_colsIn sf_3cols_3in_33" data-sf-element="Column 3">
            
        </div>
    </div>
</div>