Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
The current account in which I log into Scoutbook states that there is another user with the same email. That is probably why this account is linked to only one of my sons, a Tiger. The “other” account seems to be linked to my other son, a Boy Scout. Additionally, when the accounts are merged, please link it to my third son, a Bear.
With regard to your third son, you can connect yourself by logging in to Scoutbook and going to:
My Dashboard → Administration → My Account → My Connections
Then click on the red, bold text where it says: “Connect to your Scout as Parent/Guardian.” You will need to know your son’s BSA member ID number in order to connect this way.
Alternatively, you can contact your local council and ask them to use their Registrar Tools to add your parent relationship to all three of your children.