I was only listing my favorites.
Here's a more extensive list I found on Japan Hero (I added production companies as listed in Ultra Fan #1):
Magma Taishi (1966, P-Productions) - shown in US as "Space Giants"
Giant Robo (1967, Toei)
Supekutoruman [Spectreman] (1971, P-Productions)
Silver Kamen (1971, Nippon Gendai Planning)
Mirrorman (1971, Tsuburaya Productions)
Red Man (1972, Tsuburaya Productions)
Thunder Mask (1972, Hiromi Productions)
Ike! Godman [Go! Godman] (1972, Toho)
Ike! Green Man [Go! Green Man] (1973, Toho)
Jambôgu Êsu [Jumborg Ace] (1973, Tsuburaya Productions)
Majin Hunter Mitsurugi (1973, Kokusai Films)
Faiyâman [Fireman] (1973, Tsuburaya Productions)
Ryuusei Ningen Zôn [Meteor Man Zone] (1973, Toho)
Super Robot Red Baron (1973, Nippon Gendai Planning))
Super Robot Mach Baron (1974, Nippon Gendai Planning))
Daitetsujin 17 [Giant Ironman 17] (1977, Toei)
Megaloman (1979, Toho)
X Bomber (1980, Jin Productions)
Denkô Chôjin Gridman (1993, Tsuburaya Productions)
Tekkôki Mikazuki [Iron Armor Machine Crescent Moon] (2001)
(one of the directors was costume designer for Kamen Rider Black RX and directed Kamen Rider ZO & Kamen Rider J films)
You can see that there were once other companies producing Tokusatsu series besides Tsuburaya, Toho and Toei. Odd that only the three T's survived.