I read the novel so let me explain some things that may or may not be clear to readers.
When she taught the girls how to use magic, they cried because the MC basically greatly changed their fortunes. This is from memory, some details might be wrong. From what I recall, all three were somewhat unmarriageable or undesirable marriage partners since they were younger female children of commoner, merchant, and minor nobility families. As such they would not have a good dowry, nor offer enough in terms of wealth/social position/power to marry well.
Thier futures were either: concubine to a rich merchant or second/third wife to an old/fat/ugly noble.
By teaching them magic, those three girls suddenly became really powerful compared to other girls. Therefore, they can now marry well.