Why do the names of the Sankakusu sites match the surnames of Uranus, Neptune, and Pluto?
Only Tenouzu (天王洲) appears to be based on a real place, Tennouzu Isle (天王洲アイル), officially called Tennoz Isle in English). It was reclaimed from Tokyo Bay in the 1920's and 30's and then redeveloped in the early 1990's. The other places appear to be derived from the other outer senshi's names by extension. 

Both Sailor Uranus's (セーラーウラヌス) name and her alternate identity's family name, Tenou (天王), are taken from the planet Uranus, which in Japanese is called Tenousei (天王星). Similarly the names of the other two outer senshi are also taken from the names of planets, Neptune (kaiousei, 海王星) and Pluto (meiousei, 冥王星).

The name of reclaimed island and the Japanese name of the planet Uransus  being similar seems to be a coincidence. The planet name, tenousei, literally means sky god star, referring to the Greek god Uranus's role as god of the sky. Tennouzu Isle is named after the Shinto/Buddhist god of disease Gozu Tennou.

I doubt there's any in universe explanation of why these sites happen to have similar names as the three new sailor senshi, just as there isn't any explanation on how all of the senshi's family names coincidentally reflected the sailor senshi identities that most of them would only acquire later. I think its simply a case of taking a real world place name that happened to be similar and extending it, only implying some sort of connection. For example, while Sailor Moon's family name Tsukino (月野, literally "moon field"), suggests a mystical connection to her role as Sailor Moon and her being a reincarnation of a moon princess, it's really just a pun. Her full name in Japanese is pronounced "tsukino usagi" which could also interpreted as "tsuki-no usagi". That translates to "Moon Rabbit", the Eastern equivalent to what we in the West call the "Man in the Moon". 

Here's a screenshot of the map shown of the "Sankakusu Delta" in episode 28 of Sailor Moon Crystal and scan of the same scene from the Sailor Moon manga. If you compare that to what Tennouzu Isle looks like on Google Maps, you'll see that they placed the fictional Sankakusu and it's Tenohzu, Kaiohzu and Meiohzu districts smack dab in the middle of the real Tennouzu Isle.