Three times rent...but with (1, 2, 3!) kids?

[Charlie B.]

I've read that most landlords require a (gross?) income of three times the monthly rent.  What if the applicant has children?  Fair housing laws prohibits discriminating against parents, but common sense tells you that someone with no dependents making the same amount will have more discretionary income.  How do you account for that while staying on the right side of federal laws?