The development of Gen 3 IITs started end of the 1970 and the first contract for Gen 3 IITs (the OMNI I contract) was signed in 1981.
There are several differences between the Gen 3 and Gen 2+ technology. Main difference is that Gen 3 uses GaAs photocathodes instead of the multialkali photocathodes used in Gen 2+. Their conversion-rate is much higher then any of the Gen 2+ tubes. But they have one drawback, due to the nature of GaAs, they deteriorate much faster and have a lifespan of only about 200h. So they added a thin oxidation layer on to it, limiting its conversion-rate but extending its lifespan to more then 15.000h. This film layer usually cuts the conversion-rate in half. On a side note, the conversion rate is also given in most datasheets and is called the Luminous Sensitivity measured at withe light (2856°K) and the unit is uA/lm. (uA being the electrons and lm the light, lumens, so how many electrons do you get per Lumen) We will go deeper in to the numbers and how to evaluate a datasheet in another Sundays post.
The very first Gen 3 IIT was developed in Germany mid 70s. The US was quick to acquire the technology and further develop it. ITT and Litton became the leading manufacturers. Then in Russia, Katod started manufacturing Gen 3 IITs. Today in Germany, the country where it all started, Harder Digital is producing tubes with great performance. Recently due to corporate takeovers ITT (Exelis) is now part of Harris and Litton is now part of the L3 group.
Then there is Photonis www.photonis.com, perfecting the Gen 2+ multialkali technology with astonishing results. Tubes where one is unable to tell a difference compared to a good gen 3 tube. They do have some very distinct advantages, such as excellent highlight performance, a very small halo and shock resistant up to 500G. These tubes are widely used in Europe, both military and civilian applications. The XD-4 and XR5 being mostly used by military and the Supergen line is the civilian version, tubes that did not meet one or more of the military requirements.
Mythical Gen 4, yes they do exist, there was a short period where they where officially called Gen 4. Today they are called infinity tubes, manufactured by L3. In the late 90s Litton managed to alter the GaAs photocathode in a way so that one didnt need a oxidation film to protect it from deteriorating to fast. This meant a much better light to electrons conversion rate resulting in outstanding lowlight performance. However, they where expensive to manufacture and very shock sensitive, but they are still even today seen as the nonplus ultra of all IITs.