You have two different issues. Sharing texts is caused by using the same Apple ID for iMessage. To fix this each phone needs to use a unique ID. (You can continue to share an Apple ID for purchasing, and it does not need to be the same as the one you use for iMessage, iCloud or other services.) First decide which two phones will be changing IDs. Then on these phones, go to Settings>Messages>Send & Receive, tap the Apple ID at the top, sign out, then sign back in with a differen and unique ID.
The problem with merged contacts is being caused by trying to use the same iCloud account on all the phones. When you sync data such as contacts, calendars, etc. to the same account, the data is merged and the merged data appears on all devices logged into the account. To fix this, you need to set up different iCloud accounts on two of the phones, then migrate the data to the new accounts, then eliminate the unwanted merged data.
Begin by deciding which two phones will be changing. On these two phones, go to Settings>iCloud and turn all data that is syncing with iCloud (contacts, calendars, etc.) to Off. When prompted choose to keep the data on the iPhone. After everything is turned off, scroll to the bottom and tap Delete Account. (This will only delete the account from the phone, not from iCloud. The phone that is keeping the account will not be effected by this.) Next, set up a new iCloud account on the phone using a different Apple ID and turn iCloud syncing forcontacts, etc. back to On. When prompted, choose Merge. This will upload the data to this new account. At this point you will have three different iCloud accounts, one for each phone, with identical data on them. You will then have to go to icloud.com on your computer and sign into each iCloud account separately and manually delete the unwanted data (such as deleting the contacts from the other two phones from your account, and vice versa). These changes will be reflected on each phone. When finished you will have individual iCloud accounts, each with just its own data.
