Child support involving multiple children.

Wisconsin uses a percentage of yours and your ex husbands gross income with a balancing formula as the basis for child support. It isn't clear to me whether your oldest moved of his own volition or if there was a court order. Was the change in child support due to a court order? Did you receive notice of a hearing for modification of child support?
The scenario you have with two children living with you and one living with your ex is called "split placement" and the court will (by default, unless it would be unfair) determine child support based on the number of children, how many kids are with each parent and your respective incomes.

For example, suppose that you make 24,000 and your ex makes 36,000 per year
Based on a total of three children, the law presumes that 29% of yours and your ex's income is available for child support (for you, 580 per month and for him 870 per month).

Then the courts apply a percentage based on the number of children with the other parent (the idea is that each parent has an obligation to support all of their children, and if the child is with the other parent, then the parent needs to contribute money for that support). Since you have 2 children and your husband has one, your husband has an obligation to provide 2/3 of the money available for child support to you, and you have an obligation to provide 1/3 of the money available for child support to him:

2/3 of 870 (29% of his monthly income) is 580 per month
1/3 of 580 (29% of your monthly income) is 193 per month

since he owes you more than you owe him, just subtract what you owe him from what he owes you:
580 - 193 = 387 per month owed to you.

If there was a standing order for your husband to pay child support and he is in arrears, yes, he still has an obligation to pay that. You can file a motion with your county court to enforce that support order.
There are resources in your county to help you with this stuff. Contact the La Crosse County deparment of child support at (608)785-9564 or 785-9565.