I personally find B difficult if played the orthodox way, because my hands are proportionate to my 6'4" - 250 lb frame. They're not like Chris Squire's or anything (bassist for "Yes") but cramming 3 fingers next to each other on the fret board is only possible on my classical (2" wide @ the nut). As a number have suggested, reconfiguring ones fingers is sometimes the only solution. For A and B, I now invert the knuckle of the middle finger to cover the D and G strings, and then put the ring finger on the B to complete the shape.
The opening instrumental section of "Blood on the Rooftops" (Genesis) also contains a couple of odd / difficult ones which aren't any charts I've seen. A combination of practice and experimenting with which fingers to use will eventually solve any of them.