Tips for getting youngest to bed when 3 dc share a bedroom

[uhoh2016]

We only have 2 bedrooms (both large doubles) ds 1 and 2 (age 9 and 6) currently share the smaller room ds3 is in with us (he's 20m).
We plan to put all 3 in the bigger of the bedrooms soon but I'm worried about how I'm meant to get the youngest in bed at a decent time without the older 2 waking him or preventing him going to sleep.
7pm seems to early to put the older 2 to bed and I suspect if I try that they'll just mess about anyway totally defeating the object.
Any tips or advice greatly appreciated

Not ideal but I put my youngest (4) to bed in my bed and carry him through later when his brother is in bed (13).

I want to avoid that if possible ds3 likes to get in my bed now and I'm trying to steer him away from it and get settled in his own bed for the night

we did this. We put youngest to bed. Then the room was quiet and dark.

read stories to older 2 on my bed or downstairs. Then took them in and tucked them in, they were expected to go to sleep quietly.

For your 9 year old I would have a bedside light and allow him/her to read quietly for a while.

I think it depends on how well the youngest drops off. In my experience they are not disturbed by the older ones coming in once they are asleep.