It if doesn't give you the option to extend I would wager it's using the MBR partition scheme. It will need to be changed to GPT since MBR has a 2TB partition size limit.
If the standard Windows tools do not cope with the task of partitioning or merging a disk partition, then it is better to use third-party software. In this regard, the Partition Recovery program has proven itself very well. She can easily cope with such a task.