Case 1: Because 3.2+3.0+2.7=7.9 (yuan), 3. 1+2.9+2.8=7.8 (yuan);

A paid more money than B, so A bought three books: A, C and F.

Case 2: 3. 1+3.0+2.8=8.9 yuan, 3.2+2.9+2.7=8.8 yuan.

A paid more than B, so A bought three books, B, C and E.

Therefore, fill in: a, c, f or b, c, e.