In its annual report for 2014, the ECB said that 11 reports were received between 4 November 2014 and the beginning of 2015.
"Three of the assessed reports were considered relevant for the ECB's supervisory tasks. The remainder were found to be not relevant as they concerned national issues," the report said.
The three relevant reports relate to breaches by "significant supervised entities", the ECB said. It gave no further information on the breaches, nor the entities involved.
"This paints a picture of the SSM as a full-scale regulator that has appropriate teeth," Gerald Podobnik, head of capital solutions at Deutsche Bank, told the Financial Times.
The SSM was set up in November 2013 to supervise the banking system in Europe. The annual report "takes stock" of its first year, the ECB said.
Danièle Nouy, Chair of the Supervisory Board of the ECB, described the SSM as "a key element in supporting our commitment to delivering an intrusive and effective supervision, which contributes to the safety and soundness of the banking system to the benefit of the European citizens."
"SSM was established as one of the key components of the Banking Union in order to ensure that strong and high quality supervisory standards are applied in a consistent manner across the euro area. As an integrated European supervisor, the SSM promotes certainty in the banking sector and aims at boosting the confidence of European citizens and markets in the resilience of credit institutions," she said.
One of the milestones of the first year has been a comprehensive assessment of the banking system, including asset quality review with a macro-level stress test, Nouy said.
"Thousands of experts went through more than 800 individual portfolios, containing the credits of 119,000 borrowers. This analysis resulted in the identification of a capital shortfall in 25 banks, Nouy said.
This gave access to data "used to build up knowledge about the banks we are now supervising," she said.
"Second, and related to this, the comprehensive assessment resulted in the strengthening of balance sheets. And third, it gave the SSM the opportunity to start its supervision on a credible footing," Nouy said.
More harmonisation is still needed, Nouy said. "To deliver high quality and consistent supervision in the Euro area, the ECB needs a fully harmonised regulatory framework in the euro area, to ensure the safety and soundness of the banking system."
Nouy told the Financial Times last month that this drive towards harmonisation of rules will force some banks to raise more capital.
In October, the ECB's first stress test of euro zone banks found that one in five of the largest banks were not holding enough capital.