My 95 year old mother wants to make a one time distribution to each of her 3 sons before she dies; what are the rules and limitations?

If she distributes now, it will be treated as gift and any distribution of more than $13,000 in a year to one son needs to be reported as taxable gift. Even if she needs to file gift tax return, there is a life time exclusion limit of 1 million. If the taxable gift amount does not exceeds 1 million, she will not pay any gift tax.
She can gift up to $13,000 to any one in a year without any gift tax liability or requirement to file gift tax return.

Another alternative is to rite a will. This may be better option if she has property and the current value exceeds 1 million. Your U.S. Tax Return: Tax on Inheritances (http://taxipay.blogspot.com/2008/02/tax-on-inheritances.html)

Just to reinforce what Mukata said - if she has more than $1 million in assets, it would be wise for her to consult with an estate planner before making gifts to any individual in excess of $13K/year. She can indeed make gifts in excess of $13K - up to a total of $1M over the course of her lifetime - without incurring gift taxes. However, these gifts can have an impact on how her estate is taxed when she dies. It's best to make any gifts of this magnitude in conjunction with a good estate plan so as to avoid any unintended consequences.