As of January 2018, all three invocations of Section 3 of the 25th Amendment have in some way involved (a) the colon of a President, and (b) a member of the Bush family.

While the presidential administration of George Walker Bush will go down in history for a variety of reasons, one that's lesser remembered is that he was the first President of the United States to transfer executive authority to his Vice President on multiple occasions.  The first of these occurred on June 29, 2002, when Bush underwent a colonoscopy procedure.

Though fairly commonplace, colonoscopies require a level of sedation, and Bush was intent on ensuring that there were no questions about the status of Vice President Richard Bruce "Dick" Cheney.  Should some sort of emergency occur while the President were sedated, he wanted there to be no question with respect to Cheney's level of authority.

The transfer was announced to the press the day before it occurred by the White House communications staff.  By all contemporary accounts, the President decided to invoke Section 3 on his own, with little or no consultation with advisors; and in reading the actual text of the letter, it seems likely that the President wrote the letter himself; it certainly is written in his style.  At 7:09 a.m. on June 29th, Bush signed a letter transferring executive authority, giving it to White House Counsel Alberto Gonzalez for transmission, via facsimile, to Senate President Pro Tempore Robert Byrd and House Speaker J. Dennis Hastert.  A copy was also sent to Vice President Cheney himself, after which Gonzalez called all three to confirm their receipt:

Text of Letter Transmitted at 7:09 a.m.

Dear Mr. Speaker (Dear Mr. President):

As my staff has previously communicated to you, I will undergo this morning a routine medical procedure requiring sedation.  In view of the present circumstances, I have determined to transfer temporarily my Constitutional powers and duties to the Vice President during the brief period of the procedure and recovery.

Accordingly, in accordance with the provisions of Section 3 of the Twenty-Fifth Amendment to the United States Constitution, this letter shall constitute my written declaration that I am unable to discharge the Constitutional powers and duties of the office of President of the United States.  Pursuant to Section 3, the Vice President shall discharge those powers and duties as Acting President until I transmit to you a written declaration that I am able to resume the discharge of those powers and duties.

Sincerely,
George W. Bush

What Happened Next

Unlike Ronald Reagan's 1985 invocation, this was viewed as a textbook case of how the 25th Amendment should be implemented from a procedural standpoint.  Bush's first letter, though containing a bit of "word salad," nonetheless cites the law and specifies that Cheney would become Acting President.  The follow up letter again cites the amendment, and save what could be perceived as an authoritarian tone ("I am resuming those powers and duties") it was again a textbook application of the necessary procedures.  At the time of the transfer, some in the media speculated as to whether or not the situation warranted a transfer of executive authority.  At a glance, it seemed as though Reagan's 1985 invocation was obviously necessary, while Bush's 2002 (and later, 2007) invocations were taken with undue caution.  However, being less than a year removed from the first major international terrorist attack on U.S. soil, the necessity of having a chief executive available at all times made it a prudent, rational move on the part of the Bush administration.

Letter Transmitted at 9:24 p.m.

Dear Mr. Speaker (Dear Mr. President):

In accordance with the provisions of Section 3 of the Twenty-Fifth Amendment to the United States Constitution, this letter shall constitute my written declaration that I am presently able to resume the discharge of the Constitutional powers and duties of the office of President of the United States.  With the transmittal of this letter, I am resuming those powers and duties effective immediately.

Sincerely,
George W. Bush