I also have a low placenta on my last ultrasound. My OB told me to take Duphaston so as to aid it in moving up. I heard and thought that Duphaston is taken on the early trimesters and I've never heard of someone taking it on the 3rd. But then I trusted my OB so much. I'm just not that sure of on its effects being that I also don't know anyone personally who are given duphaston on the last few weeks of pregnancy.
I do think duphaston taken in the 3rd trimester has a great difference compared to taking it on the 1st trimester. If it's safe on the latter then you should be fine.