I say stagger their bedtimes and resolve your issue.
I have a four year old and a seven year old and they have to share a bedroom. This was a huge problem trying to get them settled in and them goofing off, the talking. Why would you put your three year old to bed earlier? Even with a nap the 3 year old should be asleep before the older one in my opinion.
I say it a good idea to put the 3 year old to bed forty five minutes EARLIER, a three year old needs more sleep then a 6 year old does and it helps empower the older one too.
I enjoy just some one on one with my oldest after my youngest has gone to bed, she has been in school all day and it is nice to just hang with her.
My youngest is in bed at 7:30 and my oldest in bed by 8:15. My son is sound asleep by the time I put his sister to bed. Of course he doesn't nap any longer but he is always asleep and it doesn't disrupt him when I go tuck my daughter in.
I have a very strict rule that if the youngest gets up before everyone else he is to lay there and be quiet until his sister or I get up!! It took me two weeks really laying down this rule and having consequences if he did wake his sister up! You have to be tough on this one but it paid off as he will get up, go pee and go back to sleep now if he wakes up too early.