I do not understand what federal tax exemption that you have written about that has a three year time period to be implemented or it is lost. If you are talking about the the amount of gain that one can have on the sale of his principal residence on a sale tax free up to $250,000 per individual and $500,000 per couple, such would not be at risk.I suggest that you consult with a tax attorney further as to the specifics of your question and the desire to sell your main residence.