If Biden omitted an entire item of income, and that item was 25% or more of his income, then the statute of limitations is six years, not three years. Finally, if the item was fraudulently omitted (i.e., there was some attempt to misrepresent or hide the item - something more than just failing to report it), then there is no statute of limitations.
That being said, the IRS will not pursue the matter because he is a high-ranking democrat-party politician. He is strictly off-limits to the IRS.