Answers

2015-10-11T12:28:20+08:00
"My first and second digits are multiples of 3"
So there first and second digit maybe one of the following: 3, 6, 9

"The sum of my first two digits is 7 more than the third digit"
"D1 + D2 = 7 + D3"

"Not a multiple of 5"
means it is not ending with "5" or "0"

If D1 = 6
If D2 = 9
D3 = ?
"6 + 9 = 7 + x"
NOTE: x = digit no. 3

15 = 7 + x
15 - 7 = 7 - 7 + x
8 = x

The number is:
698

3 digit number - CHECK
3 digits are different - CHECK
1st and 2nd digit is multiple of 3 - CHECK
3rd digit is greater than 3 - CHECK
The sum of my first two digits is 7 more than the third digit - CHECK
( 6 + 9 = 7 + 8) 
(15 = 15)
Not a multiple of 5 - CHECK

So therefore, the number is 698, it can also be 968 as well. 

I get it by combining some multiples of 3 (3, 6, and 9,) and I come up with 6 and 9 MATCHING ALL THE REQUIREMENTS.
I tried 3 and 6, 3 and 9, but they failed. 
:) 






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*EDITED*
Thank you. It really helped alot.
:D
It's nice to help you and you're welcome. I am bad at math last time XD but because of brainly... :)