What is the difference between using three year pension rule and general rule pension benefits

[MaryK4]

If you use the Three-year Rule Method, your pension is not reported as taxable income until the payments you receive from the plan equal the amount you contributed. Once you have received an amount equal to your contributions, all payments from the pension plan are fully taxable. If you use the General Rule Method, part of your pension or annuity payment is taxable and part is excluded from your income every year. 

Fir even more details see Retirement Income.