<div
  sx={{
    display: 'grid',
    gridGap: 3,
    gridTemplateColumns: 'repeat(2, 1fr)',
  }}>
  <div
    sx={{
      gridColumnStart: '1',
      gridColumnEnd: '3',
    }}>
    A
  </div>
  <div>B</div>
  <div>C</div>
</div>