I think you mean 3x of your income, not 1/3 of your income. If it was 1/3, then someone making 100k would only be able to get a mortgage loan of 33k...
Or are you saying payments on debt (mortgage payments)?
When I've heard this before it was about mortgage payments. The general idea being the money you make is broken up into thirds with the gov getting a third in taxes, the mortgage payment getting a third, and living/saving from the final third.
Or are you saying payments on debt (mortgage payments)?