Acronis True Image is available in three packs (tiers): 1 computer, 3 computers, 5 computers.
If you have a 1-computer subscription, you can upgrade to a 3-computer or a 5-computer subscription. A 3-computer subscription can be upgraded to a 5-computer subscription.
To upgrade to a 3-computer subscription, you need to purchase an Acronis True Image (Essential, Advanced or Premium) for 3 computers. Three 1-computer subscriptions cannot be automatically summed up into a 3-computer subscription.