Good news, yes you can merge all your accounts under one login
1. First decide which Three ID login (username/password) you want to keep using e.g your phone or broadband account.
2. Login to the My3 account you wish to delete (e.g broadband account)
3. Select the user menu icon on the top right of the screen then select manage profile.
4. On the bottom of the screen select delete profile.
After 24 hours you can add the account you deleted back to your Three ID.