Three of the nation's largest insurance companies have filed suit so far in an effort to block insurance department efforts to stick them with part of the bill for a $3.1 billion deficit facing New Jersey's Joint Underwriting Association.

Aetna Casualty and Surety Co., Hartford, Conn., charged in a lawsuit filed Monday that it is unjust for the state to attempt to shift responsibility for the massive JUA debt to private companies. Aetna was the second company to challenge the state lawsuit. Liberty Mutual Insurance Co. filed a challenge in federal court last week.Later Monday, Continental Insurance Co. of New Jersey became the third insurer to challenge the state lawsuit, filing an action in federal court.

The insurers hope to block a lawsuit filed by the state Attorney General's office on behalf of the New Jersey insurance department. The suit is based in part on a sweeping audit of JUA operations, which concluded last year that the JUA lost nearly $1 billion since its creation because of overcharges and sloppy practices by the JUA's servicing carriers.

The lawsuit names Aetna and 14 other private insurers that were the JUA's original servicing carriers.

Named in the suit are: Aetna Casualty and Surety Co., Allstate Insurance Co., Northbrook, Ill.; Continental Casualty Co. (a Chicago-based CNA Insurance Co. unit); Continental Insurance Co. of New Jersey; National Surety Corp., Chicago; Hanover Insurance Co., Bedford, N.H.; Indemnity Insurance Co. of North America, a brand name for Philadelphia-based Cigna; Keystone Insurance Co., Philadelphia; Pennsylvania National Mutual Casualty Insurance Co., Harrisburg, Pa.; Prudential Property and Casualty Insurance Co., Indianapolis; Newark Insurance Co., East Hanover, N.J.; Selective Risks Insurance Co. of America, Branchville, N.J.; and State Farm Mutual Automobile Insurance Co. of America, Bloomington, Ill.

The companies processed policies and claims, but did not bear any risk of paying accident claims.

State officials do not say how much money they will seek to recover, but New Jersey insurance department spokesman Lenard Karp said Tuesday that the department would seek $300 million of the JUA's $3.1 billion deficit, at the bare minimum.

Mr. Karp said the department is seeking "a six-month stay on any court action" to give state officials time to continue their investigation into the JUA's administration. An audit issued last summer turned up a $908 million loss stemming from excess fees and other overcharges by insurers, Mr. Karp told The Journal of Commerce.

Mr. Karp said Aetna's case was scheduled for a hearing in state Superior Court on Tuesday.

Aetna's action is unrelated to the new law signed last week by Gov. Jim Florio that seeks to abolish the JUA and make private insurers pay off half of its deficit.

However, Aetna's legal challenge sets the stage for an early showdown over New Jersey's ability to recover any money from private car insurers to help pay off the JUA deficit.

The JUA was created in 1983 to cover high-risk drivers, and most New Jersey motorists pay an average annual surcharge of $220 per car to keep it afloat. The new law would eliminate the JUA surcharges by March 31, 1991.

A regulatory audit found the private companies charged too much for servicing fees in the early years of the JUA. It also found that the JUA lost money because private companies sometimes paid too much for claims, charged too little for premiums, ignored simple ways to recoup some claims and did little to prevent fraud.

The insurance industry has rejected the audit's findings as unfounded, arguing that high servicing fees were later lowered and the JUA did not previously object to the way the companies handled claims.

"They're ready to lynch us," said John Hawkins, a spokesman for Aetna. ''This is not the way justice should be carried out."

Aetna is charging that state plans to fight the matter in administrative law court rather than Superior Court would result in "frontier justice,"

because the final decision would lie with the state insurance commissioner.

In an administrative law proceeding, the judge issues a recommendation to the commissioner, who then has the power to adopt, modify or reject the ruling.

In its challenge, Continental officials charged that the JUA deficit is the fault of state officials who are trying to get the companies to bail out the JUA.