Is c-diff, cellulitis and mrsa related?

Yes: Mrsa is a resistant form of a common bacteria that lives on the skin. If you have a break/cut in skin you might get infection from mrsa. Cdiff is a different bacteria that might live in your colon. When you take antibiotics for another infection like cellulitis, you can kill off good bacteria in your colon that keeps cdiff in check and allow it more "housing space" to thrive and cause infection.

Not necessarily: You can have an infection ie cellulitis caused by mrsa. Take antibiotics to cure the MRSA that may lead to an alteration in the normal bacteria in the colon causing a c-diff infection.