You will have to file a Motion in the Court asking that the child support be modified. In Michigan, it is quite common for individuals to seek modifications of child support without legal counsel. Provided that the children have been split among their parents by agreement, it should be a relatively simple case of recalculating numbers. I am assuming that Washington would also have forms available for an unrepresented party to seek modification. You may want to look at this link http://www.courts.wa.gov/forms/index.cfm. In addition, prior to modification, you may want to consult with your Bankruptcy attorney to determine whether a decrease in your support obligation would have any impact on your pending Bankruptcy.
With all due respect to the previous answer, your case has both child supoprt and parenting issues that must be dealt with. With three children and a lump sum Order of child support, it will be difficult to align the kids to conform to the order of child support, especially if there is a post-seconday support obligation. You will first have to deal with the parenting plan issues that control who is the primary custodial parent and who is the non-custodial parent. There may aslo be issues of unpaid back child supportt if you have strayed from the PP. In in King county, once you file a Peition for a support mod., a trial date is set on a shortened calendar and you have to comply with all of the local procedural rule in King County (and there are a lot) to proceed with your case. Often, when individuals try to litigate these cases themselves, trials are delayed and errors made which dranmatically effect the relief you can receive from the court if you have failed to provide the proper documentation. I usually do support mods on a flat fee basis to limit the cost to the clients. So, please consult with an atorney on the east side -- King County procedure is different from Pierce County procedure, so be sure to consult with an attorney who practices in King.