think about a box that contains three different number. We can get three different 'two digit number' . Also if we want to put '00,11,22,...,99' in a box, the box should have same number minimum two times. Suppose that a box named aab, we can get two different sheet name from. So for '00,11,...,99' we have to put them boxes that named like aab. we need 10 boxes for cover them but we also cover ab so we have 10 more sheet that had covered (ıdk if my grammer is true). 100-20=80, which means that we should cover 80 sheets more. ( and this sheets named like ab ) 78/3 = 26 and we need one box more so 27. We used 10 boxes in the past so we used 37 boxes in total.

Well you can have aba and get a solution with $34$ boxes... For example take the $34$ boxes $010, 101, 202, 303, 404, 505, 606, 707, 808, 909, 123, 321, 456, 654, 789, 987, 148, 841, 159, 951,167, 761, 247, 742, $ $269, 962, 258, 852, 349, 943, 357, 753, 368, 863$.