Confusing - yes it is! 🙂
OK, I won't spend any more time on the T-SQL side. But I haven't quite finished.
Your FieldB in TableA appears to be an FK to either of FieldA and FieldC in TableB - that's pretty bad design & a little more difficult to enforce than it needs to be
Why not create a new column on TableA which is a pure FK to TableB.FieldC?
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.