3 iPhone share the same apple id

Alrighty, this can get tricky. It's brilliant you're taking steps to sort this out and have separate Apple IDs, it may take some time but in the long term, it will be worth it.

I've recently done this with my mum's, dad's and sisters devices all sharing the one Apple ID. It's a time consuming process.

Start by doing a back up of each iPhone. This will be our safety net if something is lost down the track, you have this back up to restore from.

So, 3 iPhones sharing the one Apple ID and we want to change the Apple ID on 2 of them.

So, on the iPhone you want to change the Apple ID on (after you've completed the back up),

Go to Settings > Tap on your name at the top > iCloud

Here you'll see a list of apps. Some apps may be turned off or on in green.

What we want to do is turn anything that is on in green to off. You will get a prompt that says, "What would you like to do with the previously synced iCloud (data) on your iPhone: Keep on my iPhone or Delete from my iPhone"

-- You need to choose the blue option, "Keep on my iPhone", this will leave the data on the iPhone.

Now, there are a few that won't give you this option. Notes, I know is one, it will just say turning off Notes in iCloud will delete them. If you have Notes in iCloud on the iPhone, you need to move them onto the iPhone to be stored locally.

You can follow this process: Move notes to a different folder: In the notes list, tap Edit, select the notes you want to move, tap Move To, then choose a folder. > https://support.apple.com/en-au/guide/iphone/search-and-organize-notes-ipha61270292/ios

The other issue may be iCloud Photo Library. While you are looking at all of your apps in iCloud, tap on Photos. IF you have iCloud photos turned on, make sure DOWNLOAD AND KEEP ORIGINALS is ticked in blue.

If you have Optimise iPhone storage ticked, change it to download and keep originals and wait for your photos to download onto the iPhone locally. You can watch the process in the photos app, if you tap on the photos tab in the bottom left corner, scroll all the way down to the bottom where it shows how many photos you have, below that will be a progress bar.

You will need to wait for these to download before you can continue.

Once you've turned everything off in iCloud and you have a copy of everything on the iPhone you can continue.

Go to Settings > Apple ID, scroll to the bottom and sign out.

If you haven't already created a new Apple ID, go ahead and do so at https://appleid.apple.com

Once you have the new Apple ID ready, restart your iPhone and sign in.

When you sign in, you will get an options that says, "What would you like to do with the data in iCloud, Merge or Don't merge?"

Tap Merge.

Signing in can take a few minutes so be patient.

Once you're signed in, you now have all the data on your iPhone with the new Apple ID.

Only other issue is when any of the apps need to be updated, they're going to ask for the Apple ID that they were downloaded with. So you can either keep entering the old Apple ID password or delete the app and redownload it again with your new Apple ID.

No you cannot do that.

Apple ID is not meant to be shared.

You should wipe the 2 iPhones down and start as new.

Read here: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Setup the iPhone as new with a new valid Apple ID using an e-mail address not used in an Apple environment.

Create here: https://appleid.apple.com

After all is done, you can share books, app and music purchases by setting up a Family Share,

read here: Set up Family Sharing - Apple Support

I thank you so much for your knowledge and effort. It worked 👍🏻