How does a 3rd child effect income tax?

You can use the calculator here : http://bit.ly/qA8G6Z

Also, you'll get more allocations familliale.

In total, it will depend on the ages of your children, and how much you earn, etc, but with the tax break and family allowances, probably a couple of hundred Euro's.

Also, with three kids, you're technically "famille nombreuse", which means you can apply for a "carte de réduction famille nombreuse" (see here : http://bit.ly/qSqYfa ) that gives you discounts on stuff like train fares, and some shops.