It's tough to generalize like that, since body style (A/B/C) need not imply size and chassis. Class B's run up to about 24 feet in length and are built on van chassis. There are so-called B+ models that are nearly indistinguishable from Class C in look and size. Class C's may also be built on a van cutaway chassis with the same powertrain as a B on the standard van chassis & body. If roughly the same length, you could expect the same fuel economy and performance as the B. Larger C's, up into the 26-31 foot range, will of course be less economical to operate. The full size vans used for most B's have the same chassis and power train as is used in the pick-up truck and van cutaway chassis used in Class C's, so there is no difference in fuel economy if the weight and size are similar. A few B's and C's are built on light duty van chassis such as the Dodge Sprinter and these get excellent fuel mileage - as much as 16-18 mpg with the diesel.
Class A's don't come quite as small as B's but they do overlap the larger C's. There are Class A's starting at 26 feet. Fuel consumption in post year 2000 models starts at around 14 mpg for the smallest & lightest and goes down to around 7 mpg (older ones used to be down in the 6 mpg range). Most A's, those of 32 feet or more, are in the 7.0-10.5 mpg range.