What is her wind-down routine?
Also, are you going by her cues or are you waiting for the 3 hour mark to get her to sleep? If she gets tired by, say, 2.5 hours, you should try putting her down for her nap then instead of having her wait half an hour more. You'd be surprised what a difference 30 minutes (or even 5 minutes for some babies) can make!!
Another question... how long are her naps?