Sen. Herman E. Talmadge received a boost yesterday when three of the six members of the Senate Ethics Committee investigating allegations of financial misconduct against the Georgia Democrat said they were not satisfied with key testimony from the chief witness in the case.

None of the three - Sens. Harrison H. Schmitt (R.N.M.), Robert B. Morgan (D-N.C.) and Jesse A. Helms (R-N.C.) - rejected outright the testimony from former chief aide Daniel Minchew that the senator knew of a secret bank account in his name containing improper Senate reimbursements and mostly unreported campaign contributions.

Minchew has testified for six days that he set up and ran the account at Riggs National Bank here in 1973 and 1974 with Talmadge's approval, and that Talmadge and members of the family got money from it.

The committee must make a report to the full Senate on the hearings. But it cannot forward any recommendation for disciplinary action against Talmadge unless four of the six members vote to do so. Prior to the start of the hearings, the committee rejected a secret offer from Talmadge to settle for a reprimand. Committee officials indicated they felt such action would be too lenient.

Helms said he was not comfortable accepting only Minchew's word with no corroborating testimony that Talmadge was involved in the alleged scheme.

"I was hopeful," Helms told Minchew, "that you might be able to point to somebody else, either at the Riggs bank or Sen. Talmadge's staff or anywhere else, to give some support to the repeated declarations that Sen. Talmadge knew about the account.

"But you aren't able to do that, is that correct?" Helms asked.

Minchew acknowledged that he had no one to back up his allegations.

Minchew has told the committee that he conspired with Talmadge to keep the bank account known only to the two of them. Minchew suggested yesterday that another Talmadge aide, financial secretary Allyne Tisdale, also may have known of the account. But he said he had no first-hand evidence of this.

Talmadge has denied any knowledge of the account until it was revealed to him last year. The senator also has denied getting any money from it. He called Minchew a "proven liar, thief and embezzler." Tisdale has testified under oath to the committee that she did not know about the account while it was active in 1973 and 1974.

Minchew acknowledged yesterday that he received about $18,500 from the secret account. The figure has steadily climbed from an initial claim by Minchew that about $7,000 from the secret account went to him.

According to Minchew, Talmadge received about $11,000 from the account in envelopes stuffed with $100 bills. Minchew said he turned over $10,500 of the illegal funds to Talmadge's son, Bobby, who died in 1975, and to the senator's former wife, Betty.

"You can see the problem I have," Helms told Minchew near the end of yesterday's session. "You are the only one. . .who claims any knowledge whatsoever of the senator's knowledge of the Riggs account. You have described various transactions of the senator's son, who is not here to defend himself."

Betty Talmadge is expected to testify next week.She has told the panel in secret testimony that until 1974 Talmadge kept wads of $100 bills in an old overcoat in their apartment here.

Schmitt and Morgan also sharply questioned Minchew about his testimony on the secret account. The two expressed dissatisfaction with Minchew's claim that he placed Talmadge's name on the secret account to "insulate" the senator from the funds.

Morgan has been generally sympathetic toward Talmadge, even pitching in on occasion during the hearings to carry the line of questioning for Talmadge's attorneys. But Schmitt previously had been seen by some sources close to the committee as supporting the case against Talmadge. CAPTION: Picture, Georgia Democrat Herman Talmadge dispenses autographs as he enters Senate Ethics Committee hearing room. By James K. W. Atherton - The Washington Post