If I did understand right, you can use vcf-merge:

Merges two or more VCF files into one so that, for example, if two source files had one column each, on output will be printed a file with two column

you can use it where the input is the three files that you are interested in

vcf-merge A.vcf.gz B.vcf.gz C.vcf.gz | bgzip -c > out.vcf.gz

and you can the see what is common or unique between them

another why is to use vcf-isec:

Creates intersections and complements of two or more VCF files. Given multiple VCF files, it can output the list of positions which are shared by at least N files, at most N files, exactly N files, etc.