This is how it will all shake out.
Your wife's income (from all sources) will not included because she does not have a legal obligation to support your children.
You will receive no credit in the calculations for your step-son because you do not have a legal obligation to support him.
You will receive no credit or right of reimbursement for payments or expenses above the amount of child support that you paid. The modification that you are seeking can only be retroactive to the date of filing.
Your ex-wifes support obligation will be calculated, based on her income under the Oregon formula, for 2 children. Your support obligation calculated, based on your income (and not including your current wife's income) under the Oregon formula, for 1 child.
The 2 support obligations will offset each other and the party with the greater support obligation will pay the difference to the other.
I hope this answers your question. If there is anything that you do not understand, please let me know and I will be happy to explain further.
In the meantime, please click "Accept" so that I may be paid for my answer and any subsequent follow-ups with you.
P.S. I am frequently asked how someone can specifically request that I, personally, answer a question. Here’s how: for new questions in the future, always begin your question with “QUESTION FOR THE JUDGE.”