Room sharing with 3 under 3- advice?

Hello,

We are due to have our third baby a few weeks before our oldest turns 3. Our second child will be 21 months when the new babe is born. We are moving next month into a three bedroom home and wondering how to configure the rooms. We will keep baby in one room with us for a few months, but once he/she is ready to move out we aren’t sure who should share! Our 2-year-old is a fairly good sleeper, just has trouble going down sometimes. Our current youngest (currently 13 mos) has some dietary issues that we are still trying to isolate and will sometimes wake up screaming and upset for long periods of time. She is SO LOUD. There is no way our eldest would be able to sleep through it. They are currently in separate rooms. So we could 1) have our older two share and just hope that they are sleeping more soundly in a few months’ time (and also not get up to too much mischief together as we will transition our middle babe out of the crib for the new babe so both the older two will be in beds) or 2) have our younger 2 share so that maybe a newborn can sleep through noise better, especially if they are used to it from the start? Or 3) our oldest and the new babe could share and probably sleep well together.

With an an older child sharing with the new babe, I’m just worried about them throwing stuff in the crib like blankets or stuffies or small toys. I don’t know if 2-3 is old enough to understand how serious that could be, especially because they are so maternal and just want to “take care” of every baby in their lives.

Also, if we opt for the older 2 to share when baby comes, should we make that transition as soon as we move and sacrifice the last few months of potentially better sleep for the sake of minimizing changes in their lives when baby arrives? Or is it better to wait until baby is here and the girls are a bit older to put them together?

Lots of questions!! Thanks for the help!