Sometimes it is easy to see if shares of a shape are the same size.

The square on the left is partitioned into three same size shares, or three thirds.

The square on the right is partitioned into three shares which are not the same size; it is not partitioned into thirds.

The two shares on the right appear to be the same size.

Is the longer share on the left the same size as each share on the right?

So, the square is partitioned into three equal shares.

This means each share is a third of the whole square.

So, yes, the square is partitioned into three thirds - even though the shares are shaped differently.