The two-transformer circuit (A) very cleverly provides voltage
balancing while eliminating one balancing transformer. Either
approach should balance current evenly across the windings of the
balancing transformers. This may be a bit easier to see in approach B
versus A. The cost of approach A will be 2/3 that of approach B, and
it may be easier to find two similarly-sized MOT cores than three.
BTW, you can also use simple house-wire wound around a stripped-down
toroidal cores salvaged from 60 Hz current transformers. This may save
some space, and it's what the folks from Superior Electric use on
their balancing transformers. Inexpensive CT's can often be found at
low cost on eBay or at ham fests.
Also, do the balancing transformers need to have similar core areas,
or does this not matter as long as they don't saturate?
You'll need to use more turns on the smaller core in order to handle
worst-case voltage imbalances without saturation. Otherwise, it
shouldn't matter.