I love Harriet. It was in my final 3 for dd3 born 5 weeks ago.
We went with something else in the end but we had Harriet Daisy as our Harriet combo. I think you need quite a girly middle name as Harriet is a strong name imo. Also how about:
Harriet Lily
Harriet Lucy
Harriet Rose
Harriet Fleur
Personally I would avoid a mn starting with a vowel just in case it spells out something unfortunate if she gets married.
Good luck