A triple ko is a game state where there are three kos on the board, all in play at the same time. They are typically part of the same configuration, as in this example.
Say you have the kos a, b and c. Black takes a, White takes b, Black takes c, White retakes a, Black retakes b and White retakes c, repeating the original position with the same player to play. Under the simple ko rule, Black could now retake a again, and the cycle would go on forever.
The postition where we started with reappears.
Under superko rules White is prohibited from playing , because that would repeat the original position. Instead, white can play a ko threat, and if black responds the cycle can start again (with at , and at being prohibited).
Of course, a process going on forever is undesirable. Traditionally, a triple ko meant no result, and was considered unlucky. Several modern rulesets handle it with a superko rule, under which it is similar to a regular ko.
An example of a triple ko that is not a single configuration is a simple ko with a double ko seki that each player can use for ko threats. Also see the O Rissei - Cho Chikun triple ko in the next diagram.
Here are the first three moves in that triple ko.
In December 2005, a triple ko occured in the semi-finals of the Samsung Cup in a game between Luo Xihe and Choi Cheolhan. The officials were already discussing the possibility of arranging an extra lightning game in case of a draw, when Luo Xihe elected to give up the large (23 stone) group involved in the triple ko in exchange for compensation elsewhere. It turned out that his counting was accurate, as he went on to win the game by 7.5 points.
Honinbo Sansa played a famous historical game involving a triple ko in 1582, against his rival Kashio Rigen. The game was played in the presence of Warlord Nobunaga at a monastery in Kyoto. Due to a triple ko, the game was suspended without result. The next day Nobunaga's ally Akechi Mitsuhide rebelled, surrounded the temple, and killed Nobunaga. After this, a triple ko was considered bad luck.
On June 28th 2007, during the 14th Agon Cup in Japan, a game between Kono Rin 9p and Akiyama Jiro 8p ended in a no result due to the emergence of a quadruple ko. From the inception of the Nihon Kiin on 24th July 1924 till 30th June 2007, there have only been 19 instances of no result recorded. Of the 168 813 games played by Nihon Kiin professionals in this period, only 19 have yielded no result, averaging about 1 no result game every 9000 played. (Translated from an article in Weiqi Tiandi 2007.15). Hence if you cannot find too many triple ko games, do not get too disheartened, they are rather rare. For a list of 20+ triple kos, see Triple Ko.
 RobertJasiek: Such a configuration is not consistently called "triple ko". It can also be called "position with both a basic ko and a double ko seki". Regardless of what the ko shapes are called as shapes, their behaviour might be similar to that of a triple ko shape. So one might speak of a "triple ko behaviour" although one does not need to call a "triple ko shape" where there is none.