It was recently announced that bedrock will get 3 save slots for realms with a new ui in this article: https://www.minecraft.net/en-us/article/realms-plus–july0 , I haven't got this new ui or 3 slots on my xbox series x but I have got it the option on mobile and pc.

I assumed that it was slowly rolling out so I thought I would wait a bit but still nothing has changed.

The images show the realms menu on my xbox and pc.