The fraction used in section 77-2734.05 shall be computed in the following manner for taxable years beginning or deemed to begin on or after January 1 of the given year. The average of the property, payroll, and sales factors, which shall be known as the three-factor formula, shall be computed for each year and then combined with the sales factor only using the following percentages:

(1) For 1987, the weight of the three-factor formula shall be one hundred percent;

(2) For 1988, the weight of the three-factor formula shall be eighty percent and the sales factor shall be twenty percent;

(3) For 1989, the weight of the three-factor formula shall be sixty percent and the sales factor shall be forty percent;

(4) For 1990, the weight of the three-factor formula shall be forty percent and the sales factor shall be sixty percent; and

(5) For 1991, the weight of the three-factor formula shall be twenty percent and the sales factor shall be eighty percent.

For 1992 and each year thereafter, the fraction used in section 77-2734.05 shall be the sales factor only.