<Grid container alignItems="stretch">

  <Grid item component={Card} xs>
    <CardContent>
       // Card A content
    </CardContent>
    <CardActions>
      // Card A actions
    </CardActions>
  </Grid>

  <Grid item component={Card} xs>
    <CardContent>
      // Card B content
    </CardContent>
    <CardActions>
      // Card B actions
    </CardActions>
  </Grid>

  <Grid item component={Card} xs>
    <CardContent>
      // Card B content
    </CardContent>
    <CardActions>
      // Card B actions
    </CardActions>
  </Grid>
</Grid>

const styles = const styles = {
card: {
    // Provide some spacing between cards
    margin: 16,

    // Use flex layout with column direction for components in the card
    // (CardContent and CardActions)
    display: "flex",
    flexDirection: "column",

    // Justify the content so that CardContent will always be at the top of the card,
    // and CardActions will be at the bottom
    justifyContent: "space-between"
  }
};

<Grid item component={Card} xs className={classes.card}>