When did you purchase your lifetime subscription?
If it was years ago you are likely grandfathered in and can have all 3 Tablos active on one subscription. Search the forum for the exact date. Contact Tablo Support, they can add your 3rd Tablo without removing the other two.
Edited for link below.
https://community.tablotv.com/t/a-message-from-the-ceo-re-subscription-account-changes/