Yeah, expanding on what Earthman said, I find Keyboard to help you immensely theory-wise, as its so easy to work out on a keyboard.

I guess I see where you're coming from though, that if you work on all at the same time and you're starting from the same level of experiance in each, then it could be a bit constricting. So yeah, perhaps it would be better for you (its different for eveyone) to just focus on one for the time being and really get that down, then use that experiance to help you build up the others.