I use the same routine (music, dim room, dummy, grobag) day & night. My
DS started out doing about 6 hours overnight (like your
DD) and this was never affected by what I did during the day.
At 3 months, it might help to wrap her (
DS has only just stopped being wrapped this week!!). If she likes to be fed to sleep, you could try putting a dummy in her mouth once she drops off (this is what
DS likes). Also, try to be consistent every time you put her to bed (this is why I keep the same sleep routine, day & night - although the room is slightly brighter during the day, it is still dim).