How to transfer data from one iCloud account into a new one?
You can create individual iCloud IDs and keep the old ID as store ID, common for every user. That way you won't have to purchase everything again and still the data within your apps will remain different as your iCloud IDs are different.

Optionally, you can just create two more IDs and use family sharing with the first ID to share all your purchased content.

To transfer your data like contacts, notes etc. configure the old iCloud ID on a new device and when everything is synced just delete the iCloud ID. It should ask if you want to delete all your data from the device. Select "No". Then configure your new iCloud ID from iCloud settings and now it will ask if you want to "merge" all data.