Motion to modify child support

This has been a very long drawn out legal battle. He is hiding his income in real estate with his girlfriend which I have some proof of from a year ago but since then he has been very clever about hiding it. He and his attorney are claiming that the child support would be reduced if I file a motion to modify even though the custody will change and overnights with me will be more. She claims that his income has reduced by 1000 a month since he is now claiming not to be doing real estate business. She is also claiming that my income has increased based on the 3 months she averaged out. In addition, he has not turned in severance pay and she is claiming that I am harrassing her client when I mention that to him. I have figured up myself the amount that would be due me for back pay when I file the motion to compel for the severance pay based on past 2 years child support calculations and I have figured up the amount that should be prorated into future payments based on modified child support (adjusting for his claim of 1000 less income).

I know this is a lot. Ultimately, once I get the child support modified and if I use the tax returns for the past 3 years, can my ex come back anytime and try to claim that I am making more money or is there a time period that he will have to wait to try to modify child support? Can I use my documentation to show that he is hiding real estate income to offset any claims that I am making more money in the future so that it balances out?

To clarify more, my income averages out slightly more this past year as compared to the past 3 years, but there is a chance that it will decrease over the next couple of years because of changes in the company. I am wondering if I should just use the increased amount to avoid any more court battles or should I just use my last 3 years tax returns.

Child support is modifiable every 3 years and when 15% change in the support figure with the new number.