Hm, something I'm noticing is you tend to gravitate towards the same slender bodytypes. You ever try playing around with different proportions?
IE fat, burly, emaciated, mutated, etc?
I've found that doing big burly bastards and skinny waifish types has helped a lot with my ability to nail muscle groups and bone structure.
More immediately, middle and right are pretty good but left looks a bit stiff. I assume you were going for the anime "I'm a glum party pooper" type pose? if so, raise the shoulders and give 'em more of a slump.