Bakers semi-sweet chocolate packaging
Squares of classic Baker’s chocolate were usually 1 oz each so, regardless of the brand, if the recipe calls for three squares, you should use three ounces of chocolate. 



You can see in the image above that there are eight individually-wrapped one-ounce squares in the box. 

The box should indicate the weight for each square or rectangle. If it doesn’t, check for the total weight of the box in ounces and divide by the number of squares. So, as an example, if you have a four-ounce bar divided into eight squares, each square is 1/2 ounce. To have three ounces, you need six squares. 

Since you don’t state what your box has, I can’t tell you specifically but this should get you to the answer.