This is apparently not uncommon with various types of dementia.
It is just one of those strange and disturbing things that carers may have to get used to.
Sometimes these things come and go. My wife used to do it but rarely does now. In fact, she tends not to talk about herself, except in the imaginary conversations she is very often involved in when she uses the first person.
Do you have children? I just wondered if she could be reverting to a time in the past when she talked to them; mothers use the third person as in "Mummy is going to do that now, she is going to...." Just an idea that came to me.