Friend of 15 and 35, but not 45.

Since it is a multiple of 15 and 35, it is a multiple of $105$ which is the lowest common multiple of 15 and 35. The largest possible 3 digit number would then be $9 \times 105 = 945$ . However, this is a multiple of $45 = 9 \times 5$ . Hence, the largest integer that satisfies the conditions is $8 \times 105 =840$ .

Well ! every 3 rd multiple of 15 is a multiple of 45. As we consider 105 is LCM of 15 and 35. so 105 is least possibility , but according to question we want largest 3 digit.... , well every 3rd id multiple of 15 is 45 multiple. so first multiply by 4 so, 105 X 4=420 now if we triple the no (420X3=1260), out of limit. so 420 X 2= 840..