Three Northeastern banks yesterday set special business loan rates that will enable small firms to borrow for less than the prime rate, which has jumped to 11 1/2 percent in the past few days.

The small business rates will be 1 1/4 to 1 1/2 points below the banks' prime rate and will fluctuate with the prime.

Similar two-tier prime rates were established under the voluntary price controls set up by the Nixon administration in April 1973 Under that system the prime was allowed to float with market conditions, but the small business rate was kept lower and changed less frequently. The Federal Reserve Board said yesterday it has no plans to require a two-tier rate or to suggest allocation of credit.

First National Bank of Boston - the largest in that city and the 17th largest in the country - announced its "Small Business Index Rate" yesterday morning and was followed by New England Merchants National Bank - the third largest in that city and the 17th largest in that city and the 17th largest in the country - announced its "Small Business Index Rate" yesterday morning and was followed by New England Merchants National Bank - the third largest in Boston - and by First Jesey National Bank, a small financial institution in Jersey City.

The pattern for two-tier rates was set two weeks ago when Mellon Bank of Pittsburgh announced it was establishing a separate rate for small business. But the trend did not begin to spread until yesterday.