Git: Look at 328:17 for the definition of a Choleh She’ein Bo Sakanah (it was a Mechaber, not the Rama; the Rama added that Choleh B’chol Gufo is the same as having to lie down). The M”B 550:4 says that a Choleh She’ein Bo Sakanah doesn’t have to fast. Why would the definition of a Choleh She’ein Bo Sakanah be any different? If anything, it should be more Chamur by R’fuah on Shabbos.
Anyway, this is all L’fi the P’sak of the Mechaber (which the vast majority are still Noheg like). However, there is a not-uncommon custom to be even more Meikel on these 3 fasts because the Gemara calls them a R’shus. That is why some communities have no women fast these and I think the Minhag in Brisk is that no one fasts-they hold that it’s better to learn and have normal Seder than to fast and hurt the learning.