Ezzard out smarts and speeds him at LHW whipping shots into fosters body and ears whilst keeping safe and building up points. He'd have to avoid that left hook, I expect Foster to win one in a series. Ezzard absolutely pummels Foster at HW.
at 175, this is a decision win . I'm not completely certain Charles would win, but would bet that way
Charles survived Louis (granted, past prime) and Marciano, he would do just fine against Foster in terms of durability and defense. Not so sure about the other way - Foster isn't durable, he's not proven enough at LHW to beat Charles here and he's much worse at HW. You can't count out as good puncher as Foster, but I doubt he'd trouble Charles that much. Ezzard beat better fighters than Bob.