What I like to do for determining if square planar isomer candidates are the same is to pick an arbitrary atom. If you're like me and have difficulty rotating molecules in your head, this should be pretty quick and easy
Let's pick Cl.
Now look at the first structure and write the atoms by going clockwise.
This would be Cl, py, NH3, and Br.
Now look at the second structure (the one to the right of the first structure). Start with Cl again and try to duplicate the above sequence by either going clockwise or counterclockwise. You will find that it is not possible to duplicate the exact sequence. That means the 1st and 2nd structures are not equivalent.
Next, look at the third structure. Start with Cl, and you will see that you can get Cl, py, NH3, Br if you go counterclockwise. That means the first and third structures are the same.