I bought a 3amp BEC from fleabay and it just came in the mail. The little ferrite ring was shattered in four pieces. How critical is this little ring? Can I just substitute some other ferrite ring like from an old amperage panel meter or a thick steel washer or should I just return it?

Return the item, the ferrite is important and protects your system from electrical noise generated by the ESC. Don't even think about using a washer, it will be totally ineffective.

Kevin.