The 90-day seasoning is an FHA requirement where the current seller of the property must own it for 90 days before an FHA loan is allowed to be used to buy that property. FHA has a temporary moratorium on this rule but most lenders have overlay restrictions that have seasoning requirments. Also, this seasoning issue is not only for the 203k but for all FHA loans (203b, 203k, etc.). Check with the lender making the loan to learn about any overlays they have.
203k Contractor Directory for the FHA 203(k) Loan
I am currently in the middle of an FHA 203K sale on a HUD home. I do not know anything about a 90 day seasoning requirement for it.
There is a 90 day requirements on HUD homes that are puchased, rehabbed and resold. There are more regulations though such as value difference, abandoned properties etc... Check with a lender that does FHA 203K loans on a regular basis.