Tanegashima important? I've noticed that Robotics;Notes, Captain Earth, and Aldnoah.Zero all prominently feature Tanegashima (city in Japan). Are these series tied together in some way or is this just coincidence? (kind of like how Tokyo and Kyushu are featured in a lot of anime)
Tanegashima island is prominently featured, most probably due to the fact that:

The headquarters of JAXA, the Japan Aerospace Exploration Agency, the Tanegashima Space Center spaceport is located at the southeastern end of the island.

About Tanegashima Space Center:

The Tanegashima Space Center (種子島宇宙センター Tanegashima Uchū Sentā) (TNSC) is a Japanese space development facility. It is located on Tanegashima, an island located 115 km south of Kyushu. It was established in 1969 when the National Space Development Agency of Japan (NASDA) was formed, and is now run by JAXA.
The activities that take place at TNSC include assembly, testing, launching and tracking of satellites, as well as rocket engine firing tests. It is Japan's largest space development center.

Tanegashima Space Center is featured not only in the 3 anime as mentioned in the question, but also in:

Empire Earth: The Art of Conquest
Pokémon Ruby, Sapphire, and Emerald (Mossdeep Space Center is based on TNSC)
5 Centimeters Per Second (features a rocket launch from TNSC)