DCs sharing a room aged 3 and <1?

congratulations! I think I would use the study whilst they are both so small, your baby won't care about the size of room anyway until they are older. When your oldest dc is 5, and your youngest 2, I think I would consider then moving them into the same room.

I have 3.5years between mine, dd2 was always going to go in with dd1 but I put it off for ages because dd1 slept so badly for years! We moved dd2 in at 9 months and it's been mostly great. Dd1 sleeps so much better having someone in the room. They can both sleep through each other if they wake up in the night but settling to sleep can be....a challenge shall we say! It was easier when dd2 was going to bed first, dd1 would stay quiet if she knew the baby was asleep. Now they want to go to bed together and keep each other awake talking and singing. But they love being together and its lovely seeing that relationship develop.

Of course it's ok for them to share, but like you said they might wake each other up, but there's nothing wrong with children sharking a room.

I have 2 yrs 3 mo between mine. When ds was 6mo he went in with dd because she was a much deeper sleeper than me or dh and he seemed to sleep better next to her (because of her deep regular breathing maybe?). At 10mo he moved to a mattress on the floor instead of a cot as he wanted to be like dd. While it meant we spent a few weeks sat with them at bedtime til they were asleep it meant we didn't have a big exciting change to a bed for him when he was older and could have been more disruptive. They have always played together and even now when we have a bigger house have sleepovers in each other's rooms. When ds was sad about school closure dd crept into his bed and read him stories and cuddled him (we didn't know until we checked on them about 10pm to find them cuddled up asleep next to each other!). I think if you do it young enough then they just accept it as normal very easily.

I am in this exact predicament right now. DD1 is 3.5 years old and DD2 is 9 months old. We have 3 bedrooms and all 3 are occupied because I have the DC in separate rooms. It's very annoying and having no proper room to put guests/use as office to WFH is driving me mad. No one can stay over at all.

Mine both sleep through the night so I dont see what the problem would be really, though someone just mentioned older child dropping things into cot, that would scare me!!

they will be fine to share, probably from 9 months or so. but wait until the baby is here & you get an idea of his/her temperament and whether he/she sleeps well.

As long as they are both decent sleepers it's fine.