The bottom one has one resonating chamber with three resonating membranes.  The largest membrane is supported by a post so a thinner membrane can be used than would be possible without the support.

There main advantage of multiple membranes in one resonating chamber is that the more resonating surface an instrument has the louder it can be. If you try to use one very large membrane, it will sag unless it is thicker (lessening the amount of vibration) or is supported such as by the post on the bottom instrument.