Is there a word or phrase for a third-party contractor of a third-party entity?

Consider the following:

• I represent Company A
• Company A has a contractor that they offload work to, called Company B
• Company B has a contractor that they offload Company A‘s work to, called Company C
• Each of these companies renders services, not physical goods. In a sense, they all operate in a single supply chain, but it doesn’t fit the overall context of the relationship well.

Because of this, I would call Company B a third-party entity to Company A. I would also call Company C a third-party entity to Company B.

This post is close, but it doesn’t quite capture the hierarchy or context of this.

With this in mind, what are some appropriate options to describe the relationship Company C has to Company A? Essentially, how do I state a relationship about a third party’s third party?