Carnegie shares lost as much as 8 percent on the news, the second financial markets scandal to hit Sweden recently after an insider trading case.
The company said it had discovered violations of internal rules showing its proprietary trading department had overstated profits by nearly 55 million US dollars and its first-half net result would be reduced by about 19 million dollars.
According to Carnegie Chief Executive Stig Vilhelmson the three employees have left the company, which has set up a group to review risk management systems. The disclosure is the second scandal to hit the Swedish markets in two months.
In April, four people were arrested on suspicion of insider trading in the biggest case of its kind in Sweden. One worked at banking group Nordea and another at activist fund Cevian.