how to find exact thirds of the picture

While making previous tutorial I thought it might be really useful to do such short instruction how to divide your workspace into thirds. Never found any on dA.
Sure, you can just count the dimensions and divide them into three and find those. But this way is more cool. 
And quicker, if you're not the math type, just like me.   

Artwork used:  because it's never enough of Legolas.
Put together with help of Gimp 2.8, fonts from dafont.com

My other tutorials:
  
Thanks for viewing!