Probability that $a^2+b^2+c^2$ divisible by $7$ 
Three numbers $a,b,c\in\mathbb{N}$ are choosen randomly from the set of natural numbers. The probability that $a^2+b^2+c^2$ is divisible by $7$ is 

Try:any natural number when divided by $7$ gives femainder $0,1,2,3,4,5,6$
So it is in the form of $7k,7k+1,7k+2\cdots ,7k+6,$ where $k\in \mathbb{W}$
Could some help me to how to solve it, thanks

Hint 
Any perfect square $x$ Is 
$x\equiv 0\pmod 7$
Or 
$x\equiv 1\pmod 7$
Or 
$x\equiv 2\pmod 7$
Or
$x\equiv 4\pmod 7$
