All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is disliked for Muslims to take vows as it has authentically been reported that the Prophet, sallallaahu ‘alayhi wa sallam, forbade making vows.
Your mother took conditional vows, rendering the fulfillment of her vows conditional upon the occurrence of a certain thing. Such a vow should be fulfilled only after the thing upon which the vow-taker conditioned the vow happens.
Hence, if this woman's sons got jobs and she came into possession of a piece of land, then it is incumbent on her to fulfill her vow exactly as she had vowed. She is obliged to slaughter three goats, as long as she can afford it. Slaughtering a bullock or any other animal sacrifice instead will not be sufficient for her nor will it clear her from her liability regarding those vows. In principle, the vow-taker is obliged to deliver on his vow in the exact manner that he had vowed. It is not sufficient for clearing him of his liability to offer any substitute to the vowed object or change it as long as he is able to offer the vowed object itself in the manner that he had vowed.
Allaah Knows best.