Income may still be imputed to your ex-wife.
The operative word in the Guidelines phrase you cite is “and.” “And” is a conjunctive expression, meaning that all of the requirements of the rule must be satisfied before the action may (or may not) be taken. In your case, the child for whom child support is being determined in ten years old. Ten is greater than three. Therefore, the first clause of the “Potential income…” sentence was not satisfied.
In a recent NC Court of Appeals case (Roberts v. McAllister (2005)), the trial judge imputed income to a non-custodial mother who had a three month old infant with her new husband. Child support was being determined for the three kids she had with her ex, the custodial father. The COA upheld the trial court’s decision to impute income to the non-custodial mother. One of the three COA judges disagreed with the COA majority’s decision to impute income to non-custodial mother. But, this dissent was not directly based on the fact that the non-custodial mother’s baby with her new husband was under three years of age.
Seems to me your ex’s strategy is legally unsound. She cannot avoid her financial responsibility for an existing child simply because she chooses to have more children, and the Guidelines passage you cite says so. She’ll need some other grounds to get your child support changed.
On a related note, if anyone can explain how income imputed to a custodial parent benefits the child, I’d be most interested to read your explanation.