Bedroom split for 3 kids

[Flopsy145]

What set up would you go for?
1 DSS age 9 (EOW for 2 nights, longer over summers etc)
1 DD age 3 almost 4
1 DS age 6 months

We have a 3 bed, our room is largest but long and thin, that im currently sharing with DS until he stops breastfeeding around 1 year hopefully.
DD in smallest room although it's still spacious, recently decorated.
dSS shares his room with his dad's gaming set up, a proper man cave.

We're planning on getting a summer house situation in the garden and gaming set up will move there. My plan had then been for the boys to share and my DD keep her own room.
We're looking to move in 3 years otherwise would have done a loft conversion and added fourth room.

Is this the best set up?

All kids are incredibly deep sleepers and do not wake easily, so one child going to bed later wouldn't wake the other

I would have the little ones share the medium room and dss have the small one. Most of the younger kids I know share and don't desire privacy until a lot later. It's not just bed time, a 13 year old will want to get up a lot later than a 3 year old and not want a step mum popping in through the night if the toddler is restless.You can still use dss room for things while he isn't there, such as a desk area, or storage.

That may be true now but may not be true in a year.
But if you are determined to make the 2 boys share then at least offer to DSS to regularly review it

I'd bin the summer house and use the money to do a loft conversion (if you have the money to). That can be your bedroom and the hobby room.
Kids can have the downstairs rooms - I may even turn the long thin room into 2 rooms and have the hobby room in one of those and turn extra space in the attic room into an en suite

The summer house is brand new that we have managed to get for free, so not costing us anything

But you don't have 4 beds so that's irrelevant. You should be giving DSS a room in the current set up but you clearly don't want to. It's not his fault you've had another baby and nowhere to put it.