Question: I can't seem to wrap my head around this problem.How do I make sure that the digit 0 and digits from 7 to 9 are not included ? Provided- Repetition of digits is allowed.

Answer: 
When fixing two of the digits i.e. hundreds and tens, we get exactly two numbers from $1-6$ which placed at ones digit will make the number divisible by $3$. You can prove this by observing that there are exactly $3$ numbers of the form $3n,3n+1,3n+2$ in $1-6$.  

You are free to chose first two digits then you can have your last digit in two numbers.

So the number of numbers $=$ $6\times6\times2=72$
The trick is to note that for numbers that are divisible by 3, the sum of the digits in that number are also divisible by 3.

Start then by considering the three digit numbers that contain a 6. Since this is divisible by 3 already, the sum of the remaining two numbers must also be divisible by 3. The remaining 2 numbers can thus be (5,1), (5,4), (4,2), (6,6), (6,3), (3,3), (2,1) and so on.

Once you have the possible combinations of three digit numbers that add to 3, you then simply need to calculate the number of ways this three digit combination can be arranged.