File1        Output
ABC          A, B, C, AB, AC, BC, ABC
ABD          D, AD, BD, ABD
AB
BC
E            E

File2
ABCE         AE, BE, CE, ABE, ACE, BCE, ABCE

File3
ABCDE        CD, DE, ACD, ADE, BCD, BDE, CDE, ABCD, ABDE, ACDE, BCDE, ABCDE