3 kids, should each of them have their own rooms?

I have 2 kids (7 and 1) and am unexpectedly pregnant with our third. Our others will be 8 and 2 when their sibling arrives (no gender yet.)

Our 2 kids both have their own rooms, and we have a guest bedroom that is used probably 30 days out of the year.

The new baby will be in our room for the first 5-6 months, but after that we do not know if we should have baby share a room with toddler sister or convert the guest room to a nursery and store all the furniture in the garage? (Bed, nightstands, tv stand, tv, etc.)

It's nice having a guest room as well as we can always have a space for family or friends to crash and a "quiet room" for us to read or relax in during the day. It would suck losing that but I also don't know if it's selfish to keep the guest room over a proper nursery for baby #3.

What would you do?