I would say Washington defined the office of the President, as everything he did was te first time an American President had ever done it. That is, all of his actions were precedents for future Presidents (say that three times fast).
Adams presented some of the first Constitutional challenges to a President's authority in that he did some things that were blatantly against the Bill of Rights (Alien and Sedition Acts). This galvanized anti-federalist opposition. He also appointed Chief Justice John Marshall in 1801, which led to 30 years of decisions backing federal power over state power.
Jefferson expanded Presidential and Congressional power even though his ideology preached otherwise. By purchasing land, exploring westward and taking us to war in the Mediterranean, Jefferson also set precedents for the power of the office.
I think it is important to remember that the first presidents were going to create precedents no matter what they did. There was no clear language in the Constitution setting up the executive branch or its powers. Because of this, our early presidents had to define their office while already in it.
Of course Washington created the first cabinet as well as setting the two term precedent. He also made it clear that the executive branch would use force when needed during the Whiskey Rebellion.
Both Washington and Adams made it clear that foreign policy fell under the office of the president. The Alien Act gave the Adams power to expel immigrants. Adams also appointed John Marshall, forever changing the face of the Supreme Court. Jefferson stretched the powers of his presidency from his first moment in office. From the Louisiana purchase to his commission of Lewis and Clark he took on roles that would later remain essential parts of the president's job. His forays into controlling the budget and economy also grew his power.
I agree with what the others have said, and would like to add that the first presidents had a great responsability. Washington especially was very aware of this responsability, and careful how he used power. All of the first 3 presidents set precendents that every president after would follow, but also for how the fledgling country would develop and grow into or out of its ideals.
I think one of the more interesting ones, especially in today's society, is the last point mentioned in #3. That the President is exempt from subpoena is highly critical in terms of freedom of speech and justice, and is one of the changes brought about by the first three presidents that is very contraversial.
President Washington affected the power of the Presidency by appointing the first Cabinet, and also by his refusal to furnish the Senate information on the Jay Treaty when it was up for ratification. This last act was the first exercise of Executive Privilege. Thomas Jefferson by his direction to James Madison not to deliver James Marbury's judicial commission led to the doctrine that the President may, as President, refuse to execute laws which he considers improper. Also, when Jefferson was subpoenaed to testify in Burr's treason trial, he refused, thus indicating that the President is exempt from subpoena.
During the terms in office of our first three presidents, the powers of the presidency generally increased. Each of the three took actions that worked to consolidate the power of the office.
For example, Pres. Washington issued a proclamation in 1793 decreeing that the United States would not get involved in the war between France and Great Britain. By doing so, he was setting a precedent and claiming foreign policy as an area that would be under the president rather than Congress.
As another example, Pres. Jefferson purchased Lousiana from France in 1803. This was, arguably, an unconstitutional act. However, Jefferson took it anyway. Like Washington's Neutrality Proclamation, the Lousiana Purchase was evidence that the powers of the presidency were going to be significant.
In these and other ways, the first three presidents made sure that the presidency would be a strong office in our political system.