Is it valid for a woman to enter into a state of ihram from her home in Makkah, given that she arrived from Egypt with the intention of performing Hajj three days before the rituals began?

Since you intended to perform Hajj, it is not permissible for you to pass the Miqat of the people of Egypt without entering into a state of Ihram, because whoever intends to go to Makkah for Hajj or Umrah must enter into Ihram from the Miqat before entering it. You could have entered into Ihram for Umrah, then exited from it, and then entered into Ihram for Hajj from your place of residence on the eighth day. Therefore, since you passed the Miqat without Ihram and entered into Ihram from Makkah, you are required to offer a sacrifice (dam) to be slaughtered and distributed among the poor of the Haram, in addition to repenting and seeking forgiveness. Whoever passes the Miqat intending Hajj or Umrah without Ihram has sinned, and it is obligatory for him to return and enter into Ihram from there. If he does not return, a sacrifice (dam) becomes obligatory upon him, whether with an excuse or without an excuse, and whether he was knowledgeable, ignorant, or forgetful.