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.
If you purchased from your sisters their share in these properties, among whom is your third sister, and you paid every one of them with the exception of the third one, then you are only obliged to pay the price by which she sold her share to you, even if you did not pay her yet, because this price is a debt which you are obliged to repay, and the ownership of her share was transferred to you.
However, if you have not yet purchased her share until now, she may sell it to you with the price she likes and she has the right to refrain from selling it to you.