100以内3的倍数有99÷3=33个 :3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51,54,57,60,63,66,69,72,75,78,81,84,87,90,93,96,99
直接加呀，3,3+3=6,6+3=9,9+3=12,12+3=15，，，，，加三加三自己加，总共有33个
