If you're in California, that all should have been entered into the Dissomaster when the judge ran the numbers.
So, it may be that what you pay in child support is already offset by the custodial time that oldest son is with you.
If not, then you can modify child support, sure.
But definitely worth a call to your attorney, or a consultation with an attorney to plug all the parameters into the Dissomaster program to see what the correct child support would be.
There will only be one child support payor and one payee, but the final number should reflect everything in the situation.
So at best case scenario, you might see your child support obligation reduced for 2 years (i.e., while oldest son is with you), then modified back up to cover the two younger kids.
This is purely a numbers issue... if it costs $2000 to have an attorney do your paperwork and go to court to modify the child support, you'll want more than $2000 in relief from child support.
If your son is 16, you'll need to save at least $100/month in child support for this to be worth it, probably.