s1 = {{{3, 25}, {1, 4}, {1, 5}}, {{1, 2}, {2, 3}}, {{2, 3}, {3, 4}, {3, 5}}}Table[Table[FromDigits[Flatten@IntegerDigits@s3], {s3, s2}], {s2, s1}]{{325, 14, 15}, {12, 23}, {23, 34, 35}}
