Whether or not Marek's is prevalent is something like your company laying off 10% of its employees. it's not 10%. If it is not you, it is 0%. If it is you, it is 100%. Not a whole lot of middle ground.
I don't know that it is Marek's, an autopsy is the way to find out. If you find a tumor on that leg you have a real strong clue. Resistance to Marek's can be genetic, but other crippling problems can also be genetic. It seems a real strong coincidence if three of them were injured similarly. Like you, I'd really doubt that.
When they vaccinate chicks for Marek's they use Turkey Marek's virus. The Turkey Marek's virus does not stop them from catching chicken Marek's, it does stop the tumors from growing and causing the problems if they happen to catch Marek's. Since it is not a chicken virus, it does not give them Marek's or make them a carrier. Whether you vaccinate them or not does not affect whether they are a carrier, they just won't show symptoms if they catch it. if one of your chickens has Marek's, they all do and all are a carrier.