Yes, third railway battery (in addition to 305 mm and 180 mm) with 100 mm guns is also mentioned in book Hangon Rannikkopatteristo 1921 - 1998 by Pekka Silvast and it is marked on a map of Hanko base (by Stig Nyström), published in book Hanko toisessa maailmansodassa by Uitto & Geust.According to Enqvist it was the Soviet 10th railway artillery battery.
However, in latter book there's an article "Neuvostoliiton laivastotukikohdan rakentaminen Hankoon 1940 - 41" by Pavel Petrov. It has info that a separate artillery battalion in Hanko would include 10 batteries, 174th Battery with 3 x 100 mm guns among them. Battery with 100 mm guns arrived 12.7.1940 and it was installed in August. Heavier railway guns arrived only later in autumn 1940. There's also mention of V. F. Tributs's report that 1.12.1940 in Hanko there was one 100 mm coastal battery in open positions with 4 guns.
In Silvasti's book there a Soviet map (cited from book Sovetskaja beregovaya artilleriya 1921 - 1945) with battery 174 with three 100 mm guns situated on island of Långskär south of Tvärminne.