On January 1st, the elite trio radically changed their business model by ditching the near-unique status of Swiss private banks and transforming themselves into operations almost like any other.
Switzerland's two-century-old private banking sector has been based on rules which make the wealthy managing partners personally responsible for the money they manage for rich clients.
In other words, if the bank gets into trouble, the partners can lose all their assets, not just those they have invested in the operation.
Drawn from the elite of Geneva Protestants, in a city which was a driving force in the Reformation five centuries ago, Swiss private bankers have over time refreshed their ranks with wealthy, likeminded members from their own community.
Pictet and Lombard Odier have eight managing partners, and Mirabaud, six.
"The eight partners represent up to the seventh generation of bankers at the helm of the company," said Lombard Odier.
"Since it was founded in 1796, the company has remained loyal to its primary calling, which is to conserve, make fruitful and contribute to the handing down of the assets with which it is entrusted," it said.
Not being listed on the stock exchange, private banks are not required to publish their results, and are the preserve of a closed circle of clients.
Unlimited responsibility for those who run private banks has long been seen as a selling point for wealthy clients who want the comfort provided by such a guarantee.
But the tougher regulatory environment seen since the global financial crisis, and scandals such as the Madoff fraud case in the United States which rippled across the world's banking sector, have been a wake-up call.
In addition, Switzerland's cherished tradition of banking secrecy has been battered as governments crack down on tax cheats who stash cash abroad.
The United States has been at the forefront, and in August Switzerland struck a deal with Washington over the thorny issue of undeclared money banked by American citizens.
Swiss banks had until December 31st to decide whether to take part in a US come-clean programme to settle past wrongdoing.
Banks that do so will ward off costly lawsuits but still risk being fined in proportion to the sums involved.
According to research by consultants EY — formerly known as Ernst & Young — the overwhelming majority of banks across Switzerland's sector believe that the US-driven solution is damaging business.
Three-quarters of the banks told EY that they expect the automatic exchange of customer information to become the global standard, meaning the final death knell for banking secrecy.
EY said private banking faced the greatest pressure.
"The increasingly unfavourable conditions are currently leading many banks to reassess their business models," said Bruno Patusi, head of wealth and
asset management at EY Switzerland. The competitive pressure and the tax agreement concluded with the US will tend to accelerate the consolidation," he added.
Each of the three banks has been recast into a "corporate partnership", a hybrid status that will make it easier to compare them with fully-listed Swiss players such as Credit Suisse and UBS.
It is similar to the "limited company" structure in the British Isles, with its well-known "Ltd." label.
Complex global finance has made it hard for private bankers to feel safe with a traditional approach that puts all their assets on the line as they expand their operations.
The reform means they will only risk the funds they have invested in the bank, rather than putting all their personal assets on the line.
In concrete terms, the change means that the banks will now have to publish their results, and have also brought outside individuals onto their boards.
Pictet, which manages assets of 372 billion Swiss francs (€310 billion, $409 billion), held to the sector's tradition of discretion, simply saying that the change announced last year had been implemented.
It has added two individuals to its board.
Lombard Odier, which manages 174 billion Swiss francs, said that three people had joined it.
Mirabaud, with 25 billion Swiss francs under management, meanwhile said that three individuals had been brought in.
The three banks have withdrawn from the Association of Swiss Private Banks.
Another member, La Roche, based in the northern city of Basel, also has plans to change status this year.
That will leave just seven private banks in the association: Baumann & Cie, Bordier & Cie, E. Gutzwiller & Cie, Gonet & Cie, Mourgue d'Algue & Cie, Rahn & Bodmer & Co and Reichmuth & Co.