What are the differences between the intercostal muscles?

External: runs most superficial from the tubercle of the rib dorsally, around the shaft laterally to the ventral costochondral junction. Here it is continuous with the aponeurotic sheathe called the external intercostal membrane which directly attaches to the sternum.

Internal: the middle of the three layers of intercostal muscle. It attaches from the dorsal angle of the rib to the sternum where it directly attaches. From the angle, dorsally, the internal intercostal membrane is continuous with the the terminated muscle. The membrane then attaches to the transverse process of the vertebrae. 

Innermost: divided into three. Dorsally is the subcostalis, laterally is in the innermost intercostal muscle and ventrally is the transversus thoracis.