(1) Except as provided in subsection (1) of section 77-4105, unitary business having income from business activity that is taxable both within and without this state shall, for taxable years beginning or deemed to begin before January 1, 1988, determine its taxable income by multiplying its federal taxable income, as adjusted, by a fraction, which is the average of the property factor plus the payroll factor plus the sales factor. For taxable years beginning or deemed to begin on or after January 1, 1988, the weight given to the property and payroll factors in the average shall be reduced and the fraction shall be determined as provided in section 77-2734.16. For taxable years beginning or deemed to begin on or after January 1, 1992, federal taxable income, as adjusted, shall be multiplied by the sales factor only.

(2) If a unitary business does not have any property, payroll, or sales anywhere, then the average in subsection (1) of this section shall be the average of the remaining factors.

(3) In the computation of the factors only the part of a unitary group that is subject to the Internal Revenue Code shall be included, except as provided in section 77-2734.09.