Let me put it this way:

A differential backup contains all the changes since the last full backup. It has no reference, checks, no knowledge of what changed since a previous differential backup. When you restore a differential backup, that differential is restored in its entirety because there is no other possible option.

Hence, first option:

Restore full backup, restore 3TB of differential, then restore 5 TB of differential

Second option:

Restore full backup, restore 5 TB of differential

If you want incremental restores, you want log backups. They are incremental, each one contains changes since the last log backup.