flexeril vs. methocarbomal vs. skelaxin

Could anyone tell me what the difference is between the muscle relaxers Flexeril, Methocarbomal, and Skelaxin.  Is any one of these stronger/more effective or longer lasting than the other?  Also, does anyone know the difference between these three and Zanaflx/Tizanidine.  Zanaflex definitely seems stronger than the other three - it makes me very sleepy, but the other three do not.  I have been prescribed each of the first three for day time use; currently the Skelaxin and I take Zanaflex at night.

Thanks for any help!