Your two and three year old still nap? My two year old dropped his last nap about a month ago - bedtime before that used to take anything from an hour to two. Since he doesn't nap anymore, he's out in ten minutes.
Might it be worth considering dropping naps for the older two, having an early bedtime instead? Theo now goes to bed (and is asleep) by about 7.30 give or take 20 minutes either side.
If you feel you NEED that time they're asleep for your sanity (totally understand that!) you could institute 'quiet time' with a video, or an audio tape, or books in their rooms. Certainly the older is old enough to understand that. And the younger might just copy what the older does