Do they 'have to' share a room?
3 year olds... have NO impulse control yet, nor is it fully developed yet at that age.
Sometimes, my eldest child, my daughter.... gets tired of minding her little brother, who is 4.
After awhile, it does get 'old'... having to get their younger sibling... to be quiet or STAY in bed... then the older one can't sleep, peacefully... or well.
Anyway, my 2 kids don't sleep together. But tried once, on a lark.
It was just too irritating.... since their energy/sleep levels... are NEVER synchronized.
But my son, slept in his crib until when even 3 years old. He LOVED his crib and slept SO well in it. Then along the way, while 3.... HE said he wanted to sleep in a bed. So then we did. HE was ready.
But we did not put our 2 kids together. My Daughter just gets too irked... with her little brother.... at night bopping around.... as he falls asleep differently than her.