Filter Taxonomy reference field on Workbench Access permission
You must install Taxonomy Access Control module. Follow this steps:

1) Enable the module.

2) Go to admin/config/people/taxonomy_access

3) Click configure link for role "editor"

4) Click "Add vocabulary" link. Then select the vocabulary where you have terms "Term A", "Term B" and "Term C" and press "Add" button.

5) Click "Add term" link. Select "Term A". Configure permissions for this role and term (you should select "Allow" in every case). Once done press "Add" button.

6) Do the same for Term B

7) Do the same for Term C, but this time select "Deny" in every case.

8) Press button "Save all"

9) Go to previous window and do the same but this time for role "editor c". However you have to do the opposite: Terms A and B should have "Deny" in every case and Term B should have "Allow" in every case.

I hope this helps you! If it doesn't, you should checkout the Organic Groups module too. In case you need Organic Groups notice you also have OG Vocabulary module.