I played with the Columns block, setting it to 3 equal columns, and adding a paragraph block in each column with some text.
Do that as many times as you need rows
Then I used the CSS below to make them the same height and add the border
.page-id-xxx .wp-block-column {display:flex;min-height:300px;}
.page-id-xxx .wp-block-column p {border-radius:10px;border:3px solid black;}
result