If I'm the executor and one of the two kids that got an inheritance,
can I disclaim 1/3 of my inheritance it and give it to the other child
that didn't get anything?
As executor you are required to follow the rules of the will. IF the sums of money are large enough, the sibling that didn't get any money could challenge the last minute changes to the will. Short of a a court decision there is no obligation of the other child who received money to agree to the change.
If you were not the executor, then volunteering to give up part or all of your inheritance would not mean the sibling omitted would get the money; it would just mean a larger sum of money for the rest of the people who did receive money from the will.
Are there tax consequences with this approach? I imagine it would be
the same as if the 3rd child inherited based on the will, but want to
It would not get the same tax benefits that a inheritance gets. The IRS would see it as you receiving 3X dollars as an inheritance, and giving X dollars away. The 3X would not be taxable to you, but the X dollars would be handled under the gift tax rules. If the amount is below the annual maximum there would be no impact. If it was larger than that there can be ways to divide the gift that by spouse and calendar year. If it is huge, then you will have to include it as part of your estate which would reduce the amount you can give away tax free in the future.