The only legitimate way you can be in favor of abortion is to believe that it is not ending a life. If you can somehow convince yourself that a fetus isn't a human being, then yes, you can have an abortion for whatever reason you want to. But if you think it's human, then it's murder, even if you didn't want triplets.
Under your line of reasoning, it would be perfectly ok for her to kill the babies when they were five days old. Suppose this woman had been poor, and she hadn't gotten a sonogram. Then she had triplets. Would it be ok then to kill thes babies?
No it's not okay to kill human babies but if they aren't actual babies but merely potential human beings then you would agree that it isn't wrong to remove them for any reason. That must be what she believes as well.
At one time, there was a common folk belief (not a Christian one) that the baby wasn't alive until quickening (at about 5 months) so abortion wasn't really murder. In the 19th century, doctors discovered that the baby moves and is alive even before it can be felt by the mother.
If members of a society honestly believe that the baby is not alive, then abortion can be legal without it corrupting society. But if they know its alive, but don't care ... what's next and who's next? Every good reason for abortion is an equally good reason for infanticide and euthanasia. Societies that permit infanticide invariably see their children as mere possessions who can be abused or sold into slavery at the parents' whim.