I believe:
This means that Shape I and Shape III are also similar.
Since:
For the sake of simplicity, let's say that for every pair of similar shapes, we write them with an equal sign (=). Shape I and Shape II are similar. This means Shape I = Shape II. Shape II is similar to Shape III. This can be written as Shape II = Shape III.
The key here is Shape II. Shape II is similar to both Shape I and Shape III. Since both Shapes I & III are similar to II, they must also be similar to each other. Through this logic, we can conclude that Shape I and Shape III are similar to each other.
Bonus:
I wrote a simple equation to show the relationship between Shape I, Shape II, and Shape III.
Shape I = Shape II = Shape III
This just further proves the point that Shapes I & III are similar.