L.S.
I know you said that they both need their naps... but just from what you wrote I am thinking that your 3 year old might be phasing out of hers.
I think this b/c my three year old is doign the exact same thing. If she gets a 2 hour nap... she will either play and fuss 2 hours past her bedtime or wake up 2 hours earlier in the morning.
We also tell both of the kids (they share a room and are 3 and 1) that we don't wake up until it is light outside. They get put back in their beds until the sun starts to rise-- which is about 7am. Sometimes it works and sometimes it doesn't. I will let them go for as long as i can stand it and then get them up. It would be MUCH easier if they didnt' share a room and feed off of each other!